There is no limit. It is a basis level. Like in RRT2, there was a 'limit' of 30-40 trains. However, this didn't stop you having 200 trains. Sorry, but simply having a 1GHz computer doesn't allow you to have 2000 people.
I'm not sure what 'robbo' intended to mean by his reply. It didn't satisty the idle speculations by 'BiggieAl' who claims to be a programmer.
For the benefit of the "searchers" who come on this thread, the game has no "hard" cap on population, i.e. a code that stops operation at a preset number. It does have algorithm features which make it harder and harder to grow the population as it gets closer to and goes past 500.
What the players who are crazy insane to do nothing but build a huge population (probably because they see that as an easy way to have a huge score) don't understand is that only so many ticks can go through the CPU and it takes giga-ticks per second to maintain the units and the graphics. But even then, doing reasonable gameplay is not so much an issue of overloading the hardware as it is a matter of overloading the player's brain (attention span). When the population goes over 500, the player loses the ability to estimate what is going on. The evidence? Look for the many threads started by players whose economy collapses mid-game. Most of the gameplay questions arise from neglect or inability to watch and learn from what the people are doing and thinking.
"BiggieAl" is typical of too many players who claim to "love" the game, but then complain that they are not enabled as puppet masters for the units.