Just a few...
O, A, B, C, D, E, F, G, H, I, J, L, M, 3.0, 3.2, SC... Bloody thousands of the buggers! 935 wasn't a 911, rather it was a specific race car. However, if we're including that then we could also include the GT1 and GT1 98. There's also my personal favourite, the Flachbau
I'm being extraordinarily picky here though, a lot of those are just revisions on the basic car. If we're strictly talking numbers then it's 911, 964, 993, 996, 997, 991. There's only been three completely different 911s though, as the 911, 964 and 993 are one, 996 and 997 another, and the new 991 is the third. Trust me, it gets ridiculously complicated when it comes to 911s, and there's people out there even nerdier than me about them