14.3.1.2 Automatic Deletion of Backups

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 that control the deletion are kept-old-versions and kept-new-versions. 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. The values are used just after a new backup version is made; that newly made backup is included in the count in kept-new-versions. By default, both variables are 2.

If delete-old-versions is non-nil, excess middle versions are deleted without notification. If it is nil, the default, you are asked whether the excess middle versions should really be deleted.

You can also use Dired’s . (Period) command to delete old versions. See Dired.