You usually want to do 50-100 interviews at least before building anything, unless your idea is easy to build.
If you can whip a prototype up (check out prototyping) in less than a week, just make it and try to sell it. you'll learn more from that than customer discovery.
Within each interview page below, you can mark the date of the interview, personal and contact info for the interviewee, the length of the interview, whether it was over the phone, and any other characteristics you want to add.
You can duplicate each page for either B2C (consumer-focused) or B2B (business-focused) interviews by right-clicking each page below and selecting "duplicate." Remember, ideally you want a lot of these - so make a good number of duplicates.