The short answer is: "As long as it needs to be".
Guests will usually record for 30 seconds to 2 minutes when answering a question. So, if you are looking to create a 15 minute podcast episode, 8-10 questions should be sufficient, especially if you are recording an introduction and adding jingles.
We would recommend erring on the shorter side if possible. It's better to ask fewer, carefully crafted questions than a long series of poorly considered ones.
This will ensure your guests remain enthusiastic when giving their answers, and it also shows respect for their time.