Hi xcomfanatic20. If you look at /base/ufos/aircraftmanagent.ufo, you will see that all of the UFOs are defined with a numteam number. You can increase that to increase the maximum aliens available in a single UFO.
However, I believe there is also some code somewhere that effects how many are actually in each UFO. I'm not sure what parameters it uses, but I would very much like to see this entirely defined in scripts (.ufo files). If you are a coder, perhaps you could try to implement this? It'd be very nice to be able to better define and control the number of aliens appearing throughout the campaign -- and to do this through the scripts.
Most maps support enough spawns for more aliens, so you shouldn't have to worry too much about this.