> 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.