Why does the no of players have to outnumber the ruins? Typically you have no more than 8 players in a multiplayer strategy game and you can have at least 20 ruins on a moderately sized map.
The quests/ruins doesnt have to be a minigame. It depends on how much other stuff player have to do in the game. It can just be a bonus such as finding some gold or item to buff the army. I didnt like that in Warlords 2:
You enter a ruin
1. Most often: You kill a monster. Get a bonus which is helpful
2. Sometimes: You just die. Your hero is slain which is a disaster, at least in the beginning.
3. I cannot affect or predict the outcome.
Loosing a quest could instead damage the hero (requiring rest to come back to action) and the risk/outcome should be monitorable.