Dealing with the backups

Okay, I admit it. I’ve been using the Beta since November. It really helped me get my 50K words in for NANOWRIMO (actually 60K!). I’m an older person who was in school before personal computers existed, so I still have trouble doing certain things with software.
Anyway, I am doing a backup each time I sign off if I’ve done anything significant. Now I’ve got all these backup files. (About 15-20 of them?) Do I delete them, leave them, what? I’m afraid to delete the older ones in case it messes up the current document.
Could someone please let me know how to deal with backups?
Maybe some day I’ll figure out all the cool things everyone else has been playing with on this software. I don’t know, maybe by the time it’s in the 10th version?

I keep the last 5 or so and delete the others.

Edit: take it back. Dates seem to be uniform now. Nevermind.