A simple way to have larger numbers of religions in the game is to simply limit it to one/culture group.
e.g. if you are France and you discover Monotheism, then you get to choose from Judaism or Zoroastrianism. Your choice obviously becomes the religion you found. What this means, though, is that Germany and England-if they were in the game-now could not found their own monotheistic faith, but would have to adopt the choice of the French.
However Persia, if it was in the game, would still be able to found a religion if they discovered Monotheism, though obviously they would be limited just to the one not picked by France in this example. The aforementioned German and English civs could-of course-eschew French Judaism in favour of Persian Zoroastrianism, if they wished-or they could each try and be the first in the culture group to try and get the prerequisite techs for one of the other religions.
Hope that made sense.
Yours,
Aussie_Lurker.