As a test, I set autosave frequency to 1 minute and max versions to 5, and here's how it works in 6.0.2:
(Alan covered most of this correctly already so apologies if I'm repeating)
It saves these project instances into the auto save folder: 222-1.prproj, 222-2.prproj, 222-3.prproj, 222-4.prproj, 222-5.prproj
When the max is reached, it starts at 222-1.prproj agfain and workes its way down the list, over-writing each time.
You can verify this by looking at the 'modified' time for each one. There were some points where it looked like it hadn't written a project in a while bit it was because I was looking at the 'created' time which of course woule be older once the list starts being over-written. Also, I noticed that when I opened an auto-save project (e.g 222-1.prproj) a NEW auto-save folder was auto-created inside the parnet auto save folder containign the new auto save isntances, named as such: 222-1-1.prproj, 222-1-2.prproj.
That said, I'm not trying to talk anyone out of the fact that they may just be encoutnering a bug, but understanding the designed behavior will help us decipher what is or is not a bug. If things are still not working for you as expected, then either you have a system-specifc bug (which seems unlikely) or you have some other steps that you've introduced that I'm not following in my tests. Please let me know me know if the latter is the case.