I do believe there is a point of having too many champions. Let's take a crazy number like 5,000...that would be too many. We could pull that back to a much more managable number and still feel like that is enough. My thoughts are that over time we will slow down champion production, so that all players can get to know all champions. That being said, as we grow our champion pool, we will gain more options of who to update and remake.
So while I don't believe that there is a single magic number, I do believe we will slow down a bit as the years go on. Though I do not foresee this happening any time in the near future!
Perhaps, I could definitely see Nasus and Renekton getting another Egyptian themed character to roll with. Probably someone of the caster variety.