Question: How many popes are buried at St. Peter's Basilica?
Answer: There can be many answers depending on the source. Many popes may have originally been buried in St. Peter's Basilica, but their tombs no longer exist or they were transferred to other sites.
This Vatican list shows at least 49. (if I counted correctly?) This may not be a complete list.
http://vatican.com/articles/popes/the_tombs_of_the_popes-a41