To prevent unlimited consumption of disk space, Emacs can delete numbered backup versions automatically. Generally Emacs keeps the first few backups and the latest few backups, deleting any in between. This happens every time a new backup is made.
The two variables
kept-new-versions control this deletion. Their values are,
respectively the number of oldest (lowest-numbered) backups to keep and
the number of newest (highest-numbered) ones to keep, each time a new
backup is made. Recall that these values are used just after a new
backup version is made; that newly made backup is included in the count
kept-new-versions. By default, both variables are 2.
delete-old-versions is non-
nil, the excess
middle versions are deleted without a murmur. If it is
default, then you are asked whether the excess middle versions should
really be deleted.
Dired's . (Period) command can also be used to delete old versions. See section Deleting Files with Dired.