How many troops should you keep in evony?

It depends. Defending: If you have been colonized before and te enemy had 30000 warriors and 30000 archers like what happened to me, you would want an army of about 40000 warriors, 3000 ballistae, 2000 catipults, and about 10000 archers. You also want wall defences.

Attacking: If you are attacking a village you will want about 5000 warriors. If it's a town about 10000. Fort 40000. City 100000-250000. It all depends on what troops they have.