Number of experimental questions?

To answer your first question, to the best of my knowledge, the GMAC hasn't released official information on this, and there are conflicting statements from various reputable sources (i.e., major test prep companies). At least one respected company states that there are exactly 11 experimental questions on the verbal section, but that's not a universally held position. I think it's enough to know that somewhere around 10 or 11 questions will be experimental on each section, which isn't very precise, I know, but that's ok, because (to answer your second question) it doesn't do you any good at all to know that. There's no way to discern between scored and experimental questions, and you shouldn't try. The only possible value to knowing about the experimental questions is that if you know that you absolutely got a question wrong, you can at least comfort yourself with the thought that there's approximately a 25% chance that it didn't count. That can be kind of soothing; otherwise, though, worrying about experimental questions is a waste of time, at least in my opinion.