There is practically no possible reason you would need to count that. I doubt that you are generating the method dynamically at run-time and do not know the count (in which case you could still track it). Just count how many "cases" you wrote.
Sure there is possible ways to psuedo-count, such as only using consecutive numbers, and iterating a range calling the method until it returns nil, but that is a god-awful practice that you should never, under an circumstances get in any habit of actually trying to do.
To answer your question, no, Ruby does not offer any way of counting the "cases" in a method, mainly because it does not make sense and is pointless. Just out of curiosity, what exactly do you need that for? I guarantee that you simply need to change the way you are doing something if your code requires such a thing.