The reason Civ3 only has 31(2) civs is because of performance problems resulting in the slow games everyone complains about. Even adding a few more than 8 civs apparently already causes a jump in turn-rate lag.
From what I can tell, Civ4 will have double if not triple the requirements that Civ3 does. So if anything, the limit is performance-related.
Although, AFAIK Firaxis has only said that there will only be 18 civs IN THE VANILLA GAME. I don't think they've confirmed that a higher number won't be supported for modding purposes (i.e. that you won't be able to ADD more civs). [Pardon me if they have in fact said that 18 civs is the maximum limit--I just haven't read it anywhere...but then, I haven't been keeping much track since they siad they'd be using Python (which means that there won't be much in terms of limits otherwise).]