My current reality is that P2P allows for a higher maximum number of player than Server-Client does in the case we don't use dedicated servers
Actually, the one crucial assumption is that everyone has the same upload limitations. If that is true, then P2P will allow larger amounts of clients than C/S -- but only up to some small amount of clients, like 16 or 32, because limited upload is, well, limited :-)
As soon as you can find one user who has better than average upload, then C/S gives you a lot of benefits, including an authoritative server, larger player counts (even for players with much more limited upload), etc. And that server is, generally, a normal playing doing "hosting," not a dedicated server paid for by the game publisher.
When it comes to RTS games, I'm not so sure they are all P2P -- if you look at Starcraft, for example, it has a clear host, and when on the Internet, it goes through battle.net to avoid NAT problems. I wouldn't be surprised if it actually was C/S topology with the "host" as the server. However, it doesn't matter much, because with 4-8 players, neither model is all that different in network bandwidth usage, so other implementation worries probably weigh more heavily.