How many questions can I ask?
Ask as many questions as you'd like! Just don't ask unrelated questions in the same comment; instead, ask separately, so they can be moderated separately. (More chances to win!)
I asked nearly the same question as one you selected to pass on; why?
Often several readers ask similar questions. If yours is the one Slashdot editors believe represents that "group" of questions best, it's the one that gets picked. We tend to pick questions that have meaningful subject lines, and we prefer questions that are short and pithy to long, wandering ones.
Hey, that eleventh question wasn't even highly moderated!
It's a human factor. Moderation helps us decide which questions to send along, but often there are interesting ones which haven't been modded up as much.