Why can you only found one? Are we talking about some game opion here? If so the AI for that is Thunderbrd's responsibility for DP and CR. I thoguht we were talking about the AI researching already founded ones in he standard ruleset. I was anyway (to clarify the context of my posts)
I always lately play with limited religions and divine prophets on. So one civ can only found one religion unless it loses Holy city.
Probably AI code should be tweaked while using those 2 options.