Is there a Unit limit in the game? I had plans on buying the game but unit limits turn me away because I love massive battles.

if it does have a unit limit, is there a way to mod more population in? thanks.

I've just check the manual and there is a section about army limit but it seems you can increase it as much as you want. You get:
+ 5 units for each level of Keep (levels 1 - 4)
+ 10 units for Keep level 5 (maximum)
+ 2 units for each standard building (not including walls and resource building)
+ charisma bonus
Building more Keeps doesn't help you - only the highest level Keep is taken into account. However, you can build as many standard buildings as you want and it seems that it will always increase the army limit.

That is indeed so. It is part of what made this series so much better than many other RTS games. You can build a truly massive army.

Check for the cheapest building of your race and build a LOT. That is what they are there for, even if they don't provide you with anything usefull otherwise, they do up the army cap. And towers are always good of course.

Also note that captured buildings up the cap just like ones you build.