There are several sources.
a) Uses the same numbers as you may find on ATP's Meda Guide, but is updated. Though it only goes back to 10 titles. I haven't checked the reliability of this site.
b) Made by myself collecting results from the tournaments mentioned in ATP Results Archive only. Not fully updated. Both ATP Result Archive and ATP Media Guide have lots of errors and missing tournaments. Look in my signature [ATP Titles confusion] for a thread about the differences. I have to admit I think the Media Guide numbers gives more sense. If you want a list that goes back to 1 title please use this one
c) Reliability zone has even more errors. It counts World Team Cup as titles, which should not count as individual titles. I don't recommend this one.