When does PGP create a new xxxring-bak-x.xkr ?

I am using now PGP 6.5.8 for quite a while. But I didn't found out until now
when PGP creates a new bak file (with a name pubring-bak-x.pkr or secring-bak-x.skr)

First I thought it will be created when something changes in the keyrings but this is not the case.
Currently I have 4 bak's for pub and sec each. When I delete one of each of them the missing
keyring backup is created regardless wether I modify keys or not.

Should there always be 4 backups?

Peter