I had exactly one question on probability and counting on my last real GMAT, and it was a completely standard question. There is a myth that probability and counting is somehow more heavily tested at the higher level of the GMAT. It's not; probability and counting questions can be easy, or they can be hard, and they're no more likely to appear at the 700-level than at the 400-level. On occasion, people encounter several questions on the topic, but that is unusual. You certainly want to know the basic principles of counting and probability so you aren't in danger of answering an easy question incorrectly on the test, but I think many test takers devote far too much time to studying highly specialized counting situations that are extremely unlikely to appear on the GMAT and too little time to studying the topics that are absolutely guaranteed to show up many times on test day: ratios and percentages, divisibility, inequalities, etc.