Each unit of stormtroopers has 2 squads of 9 soldiers each in the out-of-box game. If you land 10 squads of stormtroopers (or 10 AT-ATs), you'll have 180 troops on the field. If you also land General Veers, who doesn't take up a population slot, you can drop an additional 18 stormtroopers for a total of 198 soldiers.
The Rebel troopers come in three squads of five per unit; so each unit of Rebel troops has 15 soldiers. If the Rebels landed nothing but regular infantry on a planet, they could have 150 troops.