Each Empire is currently allowed roughly 133 troops per continent, or 13 full squads.
Population Lock status can be viewed from the Interstellar Map as a padlock symbol on one of the side windows.
During the year of 2003, population capacity was not evenly split between Empires. Each side could potentially fill up nearly two-thirds of the slots, leaving one-third to split between the other two factions. It became standard practice for an empire to rush a continent to capacity to gain a permenant number advantage over the opposition.
During the year of 2004, population capacity was evenly divided between each faction, allowing roughly \~166 troops per side to fill a continent.
For 2005, population capacity was reduced to its current \~133 per side per continent, this created the side effect of more fights across the galaxy.