OS3 had 5 betas.
I believe OS2 had 4 betas, that's all I could find on Google.
With 3.0, it was 8 days between Beta 4 and Beta 5. Then it was 33 days between Beta 5 and GM to devs. I don't think we'll see another beta, just GM on June 7th.
Regardless, I don't think we'll see another beta, especially since the phone is rumored to be out mid June at the latest. Whatever build Apple has internally is probably already being loaded onto the phones in production. They won't release another beta and then wait for testing for a GM. The GM has already been declared internally. We'll get it after the keynote.