The maximum amount of troops on the battlefield is determined by a setting in the options menu, with a maximum of 150 total. the ratio of troops on either side is determined by the total amount in both armies and the relative tactics stat of both.
For instance, with 200 solders, fighting a group of 100, with even tactics scores you would get 100 men on the battlefield and they would get 50. fighting a thousand, you would get 30 or so to their 120. Once the ratio has been changed, (by death) then each side gets reinforcements at gradually slowing intervals until the unused fighters are used up. If an army is entirely defeated before the reinforcements arrive again, another battle is fought, until one of the armies is exhausted.