bookmark-version-control

bookmark-version-control is a variable defined in `bookmark.el'.
Its value is

nospecial

Documentation:
Whether or not to make numbered backups of the bookmark file.
It can have four values: t, nil, `never', or `nospecial'.
The first three have the same meaning that they do for the
variable `version-control'; the value `nospecial' (the default) means
just use the value of `version-control'.

You can customize this variable.