True. That is really difficult to obtain though. But assuming that most people are eventually diagnosed with cancer (even if just close to death), then a similar maybe a little lower population must have undiagnosed cancers.

In the end you'll need a properly conducted study with random undiagnosed people to get reliable numbers though.

It seems like pretty basic info to me. There should be some estimate out there somewhere, especially given the billions of dollars spent each year on studying cancer. A quick search didn't lead to anything promising though.