Well it depends on what the average Battleship size is and what amount of resources a species or space nation has to spent on battespaceships.
And older ships don't disapear. There are some option to handle older ships.

Well Earth made it's baby steps so not a theme for a space RTS.
But a fictional species. That had a realy long time ago, made there baby steps. And after that still a long time ago starting kolonising and mining there sol system and kolonise even there neighbore stars and there planets. There space era is like several centuries long.
So they have a lot resources to spent for many years.
If they use 1/4th on BShips it could be a couple of 100 compararable to naval carriers size. As a crude guessing. or 100 modern 60 previous gen and some older still in service.

If you look at starwars with there insane sized ships that unreal. You need to have a lot of Stars and there planets and roid minded to deliver such resources. It's not impossible, but the empire must be realy big.

BSG had lot's BShips. But got there ass kicked by cylons. Kolonials have a lot of kolonies, planets and so multiplanet resources. So lot of those Battlestats.
But I miss different classes and also the evolution that classes go true. Due to the series focus on a older class you know there is at least a new modern class Pegasus. The back story isn't there, if there are more classes.

Startrek. Due to manny races you get many set of ships.
each race have there own list of ships.
And the federations use ship classes. Wich is more a form of naval sheme. You have some shipyard. And a big ship produktion cycle is long. In time tech advances and new better designs and adoptions comes in. Also produktion cycle aren't synchron. So let asume buildig a ship cost 1 year. With 5 ship yard somewhere in a production cycle. Each 2 years there come new designs so new class. Each year adoptions and tech advances, steady each 9 months, so a variant of a class.

So you get something like this, a space nation with 8 classes where the older once are retrofit and modernised. To old are broken down for resources and each clase have 3 to 7 ships build, with 2 to 3 variants.

Lets say 32 BSships.

So with 7 species you have at some point 150 to 400 ships where there are 1 to 9 of the same type

Because there can be different roles for big ships there is a need of different classes. so you can have 5 current classes. And lot of older once.

Wenn encounter a alien race on war path.
Classes don't match.
Types that did well in other inter planetary wars. Could be a bad thing agains this new foo. And are not produce anymore. But after some years of losses a class to adress this comes up. Current class might get a heavy update to adopt to new effective tactical choices needed agains this foo.
There is a armsrace and there are ships build that works well and better classes design to repel enemy types better.

So it's a natural thing in war to get newer classes.

This means lot of different big ships.

The problem I have with space ships in games scifi series is there unrealistic conctruction. Some ships would break if gravity get a hold on them. Or wenn they have to avade at full power. In real life.
Like the adromeda spagetti design.
Wasp boddy shape of TOS Enterprise.

So if you go for realism. There is no balance. There is a arms race and the goal is to have the better ship. It means also a lot of diversaty wich consume Content creation resources of a dev team.

This problem is that you have one gameplay style for a game that grows on you and become something different.
The problem is the fases of the game.
RTS there can be a large difference between the fases.
Like start game, mid game, end game.

In RTS you start with a few ships.
in this fase you can micromanage one to 3 ships. So the gameplay is aimd at costumation heavy and techtree.

In mid game the gameplay shift from Captain of a lead ship to large fleet admiral play. Where you handle a large feet. Here resources and logistics and customisation must be handled more automated. Because as admiral you have not the time to micromanage a fleet of 16 Battle ships and escort and support ships. The tactical command gameplay take a lot of your attention.

Wenn you get to a fase where you have reach Space Empire status. Lots of resources. The gameplay must shift to a higher level of abstraction.
You handle fleets, so actualy admirals

Optional you handle the composition and size of fleets and there armanent.
You build fleets. Not ships
You can build even bigger supereorityships then before.

The problem is most RTS is that a Emperor must micromange his whole empire it self. If his wariors are dodo's

Automation on many levels of core gameplay lead to a complex and deverse AI requierment. Could be why this fase is ignored.

The game change from ship customizing sheme to a fleet cusimizing sheme.
And Battlstar command menu to a more fleet command menu.

I do far prefer having less designs I must say. It just makes sense.
In real life you don't get one individual ship being made very often; you get a class of identical ships being made.
Even if you're making a totally new class of ships it would still be based upon principals established with earlier designs. The entire reason you can now make super advanced ships when you couldn't 10 game years ago is that your engineers and scientists have worked with what they had and figured out how to make it better.

time scales in RTS are always screwy anyway. Your talking lots of "game years" between ship design improvements. Sure the actual combat in the RTS makes it feel like it is at normal time speed we are accustomed to, but the resource gathering and construction are highly compressed time scales.....or do you really think it only took 2-3 mins to build that super duper giant battle cruiser? so an RTS is kinda operating on two different time scales. so the resource gathering/ship construction is probably taking a long time between new ship designs if you go by its highly compressed time scale.

Quote:I completely agree with your point about expenses; space is not cheap.

Actually, once in space, it is not that expensive. What is expensive is getting into space (and if you have to lift all your life support materials as well, that makes it a constant cost).

We currently need to lift all life support materials into space each time, this is what the major cost of a constant presence in space comes from.

If you could mine asteroids, and get everything you need form them (oxygen from the water for life support, hydrogen from the water for fusion power generation and fuel, carbon for organics, metals for construction, silicon for electronic, etc) then as they are already in space, you do not have the costs for getting them into space.

Also, using rapid prototyping technologies you could construct space ships from the asteroids as well. If the ships you build have these rapid prototyping factories in them (there are rapid prototyping machines that exist today that are about the size of a desktop printer and cost under $500 - look up RepRap) then they would effectively be Von Neumann probes (http://en.wikipedia.org/wiki/Self-replicating_spacecraft).

There are massive amounts of material in the Oort cloud, or, it might be possible to dismantle a small moon (like the moons of Mars) and turn these into ships using Von Neumann probe concepts.

Remember, each ship is capable of constructing more of its kind, or even other kinds. You could even have each of the individual ships capable of constructing factory ships, which then in turn can construct any of the smaller ships (a Von Neumann probe ecology).

A variant of the VN ship is the seeder which stores genetic material and has the ability to grow that material into an organism. This seeder could contain the genetic material for a human that can become the crew of the ship.

If crew are lost, this genetic material can be grown (there might be many in different states of growth) and these could replace the ones that were lost. They could even be cybernetically incorporated into the very structure of the ship (they are the ship).

Given this scenario, it becomes feasible or there to be millions (if not billions - or even trillions) of ships in a system. Each one capable of self sufficiency, and even rebuilding the entire fleet. With sophisticated command and control systems (based off of colonial insects for example), the whole fleet could be co-ordinated as needed.

This kind of scenario is feasible with technology only a few more years advanced than what we have now. In fact, most the technology exists, it is just not to the level where we could do this.

So, when it comes to realistic, this is a much more likely scenario than Star Trek, Battle Star Galactica, or Star Wars. All these assume that production technologies do not advance from the 1960s, except in the size and location that they are working in.

From a game player's point of view, however, they will likely be in control over a group of these ships. So if each ship had a level of automation, the player just needs to select a target and they should do the rest. Including dealing with any incidentals on the way, like other enemy ships encountered, avoiding attack and such.

A Skirmish, might involve thousands of such ships, and an massive battle might involve millions. And, if we are talking about interstellar empires, rather than just a single system, you could be dealing with trillions of combatants.

Have a read of a short story by Alastair Reynolds called "Spirey and the Queen". It is set within such a "war".

Looking at Orion I’m reminded of Galcon on the iPhone. In this all options are reduced to the minimum, you have one ship and construction is automatic and based simply on planet size but it can be a frantic and pure strategy game.