> how many permutations of different questions are there?
Infinite! Just this of the possibilities with that kind of system. You could keep adding different questions (just minor variations) and it won't be difficult to keep ahead of spammers. Plus there is always a trade off between security and ease of use. I think the ease of use with clickcha is well worth the slight loss of security.
Having to manually add questions means that practically speaking the number of possible questions <<<<<<<< infiti. As soon as you add another question, the spammers will simply add it to their portfolio of question types. There is no way "you can keep ahead of the spammers" with this.
And then you fall into the same problem with a race between captcha/clickcha programmers (who MUST maintain ease for human-use) and spam programmers (who can click randomly and still sometimes succeed, so they only seek to boost their chances).
I'll give them two guesses who will win most of the time.