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.
With regards to Blind Gate, since the spell decreases population, and then increases again once it times out, leaving an Earth Elemental, it is possible to glitch the game and go over the population cap of 80 by purchasing units to 80 population while the Blind Gate is still operational. Once the Blind Gate finishes, an Earth Elemental will be returned and contribute to 2 population.
V has the highest population among all units which is 10, even Giants have population of 7.