I've heard that most exam banks contain a little over 150-200 questions, although I have no scientific evidence for this assumption.
I know a few people who retook the Sun Certified Java Associate (SCJA) exam (because they hadn't read my books before taking it the first time), and they said that about 1/3 of the questions they felt they had seen before. So, with a 50 question exam, that should equate to a bank of just under 300 questions. Of course, I'm a Java dude, not a statistics major, so I may be off.
I've got over 350 exam questions in my SCJA Questions Guide, which is even bigger than the bank - now that's value!!!