A simple, but very effective phpBB antispam tool Textual Confirmation (TC) asks newly registering user a question. If the answer is wrong, TC rejects the registration.
How much questions do you need for the best protection? Hard to say, but definitely not 50.
Earlier or later, a cheap outsourced monkey answers some of your questions and adds the answer into the spammer’s database. As a counteraction, you need to change you question. When you have 50 questions, it’s a tedious task.
In my opinion, 2 or 3 questions is enough.