Statistically, how many past US presidents are likely to have been gay?

I do not think there is an easy answer to this. It is possible there were some from a period when it was FAR more taboo than it is today. The problem is that POTUS is a job that not everyone wants. So there is a lot of self selection there. A qualified gay person may have chosen to NOT campaign because being POTUS means that your private life is no longer private. If you become POTUS the press follow you around as much as possible. Self selection is something you must consider in a question like this.