10? I always prompt prepare a document with questions that will inform the technical brief for the task described above. The end result is between 30 and 70 questions most of the time. And 95% of the questions are valid, i.e. they really help describe what I had in mind, they are either questions I did not think about and would implicitly answer during implementation, or decisions I had already answered in my mind but clearly could be answered in a different way.
Its very useful even for just making your mental model concrete and documented.
And that is just the first round. After I am done we have another round about new concerns that emerged and then a third one typically.