Depends on battle size. Say you use a 200 battle size - it should be around 100 for each side (not counting tactics bonuses), so from the 200 men army, only 100 will be fielded at one time, and out of that, it's all calculated according to the lords' bonuses, so each gets a portion of soldiers - your tactics skill and overall manpower within the army is not that huge, so you only get a small portion of your soldiers with you. When X amount of soldiers are incapacitated, a reinforcement wave should arrive, and proportional soldiers will arrive with more men - say you had 10 men, now 8 of your men come around, as well as 30 from an ally lord, and so on until all the reinforcement waves are depleted, an army retreats or all the soldiers over the 200 battlesize have been called to the battle.
It should be proportional according to the battlesize you choose, so have that in mind. And remember to config the amount of reinforcement waves you want (1-5 or infinite, I think are the settings). Having endless waves is very cool, but makes the battle feel longer and your health may not be enough to survive it, so keep it in mind. Also may take quite a hit on your pc. But having 5 waves might be good for those 5kx5k battles, where you have a gap to regroup, change the outcome of the battle or even save/run away accordingly (less realistic, maybe, but useful nonetheless). That's up to your tastes, though.