Population acts as a limiter that prevents armies from getting too large to prevent lag and as a balancing measure. It is represented by a small stick figure at the top right corner of the screen with a number beside it.
Almost every unit has a population value. The population value of all of a player's units is added up. This number is not allowed to exceed 80, although some units do not cost population (see Trivia). A unit will contribute to the population count if it is either alive or is in the process of being built.