The maximum units a player can have under their control at a given time is 20.

Before patch 1.2, any units in the field over and above 20 were controled by the AI (or if you had two armies of 15 units, you would control 1 army, the AI would control the other).

After patch 1.2 you have the option to carry on as previous, or allow drip-ffed reinforcements. This means that if you have two armies of 5 units, one will start out under your control, and 5 of the re-infocement units will come into the battle under your control, and then as you units are killed or routd, new units from the re-inforcements will be come into the battle ,under your control, upto the 20 unit limit.

Thats as I understand it at least.