You never want to have more troops then you can feed or they will desert you.
Each troop takes a certain amount of each resources and population to build as well as certain research requirements. For specific requirements on each troop look in your barracks under that troops tab. The reqs in red are what u need still to be able to make that unite. The best combats unite for might building is archers. They giver 4 might and cost 9 upkeep per hour while the best non combat unite is supply wagons with 6 might and 10 food upkeep.
might and food consumption
Supply troops 1 might - 2 food/hr
scouts 1 might - 5 food/hr
militia 1 might - 3 food/hr
pikemen 2 might - 6 food/hr
swords 3 might - 7 food/hr
archers 4 might - 9 food/hr
cavalry 5 might - 18 food/hr
hvy cav 6 might - 35 food/hr
wagon 7 might - 10 food/hr
ballistas 8 might - 50 food/hr
rams 9 might - 100 food/hr
Another thing to consider when attacking is your troops speed. The way the computer runs combat is pretty simple each unite get 1 attack per round till they are dead. Troops also go into battle in a certain order.
Troop Speeds: scout.........Speed:3000
first of all never send scouts on an attack, period; scouts are for scouting. i have noticed when attacking a target speed is a critical factor. once the army arrives at the target, it's as if they all line up parallel to the opponent and then charge into battle. this is why scouts will always be the first to engage and will be slaughtered. Light and Heavy Cavalry will be the next to engage in combat. this is why you invariably see huge losses to those troops even when no spiked barriers are present. Pikes and Swords are the next to engage but do not suffer as many casualties for they are quickly followed by archer cover. Militia will suffer the fewest losses as they bring up the rear and hide behind all the dead bodies of their fallen comrades.