I’ve said it once, and I’ll say it again: if you have a chance to go hear an author talk, do it. It’s fun, it’s interesting, and at the very worst, it’ll teach you what not to do (unfortunately, not all authors are decent public speakers).
That said, one of the things not to do as an audience member is to monopolize the author’s time by asking question after question. There are other people there, too (we assume). It is not a private interview. Asking one or two questions (with a space in between for others) is acceptable in the Q&A portion of the talk – even encouraged.
Now, if you run into an author at a bar, you can ask all you want. Just don’t forget that he/she has the perfect right to tell you where to shove your 12th rapid-fire question (or 9th, 3rd, or 23rd – it depends on the author) and get up and leave your intrusive badgering. So it’s probably a better idea to try to carry on a conversation like a regular human being and insert your biggest questions on occasion within that. (You know, subtle-like.) Which will also likely limit the number of questions that you get to ask.
So… here is my question for you: If you could only ask an author 2 questions, what would they be?