Used as nnum action for multilog, as set in various /var/qmail/supervise/<module>/log/run files.
The default Qmail-Toaster setting is 100.
man multilog excerpt:
nnum sets the number of log files for subsequent dir actions. After renaming current, if multilog sees num or more old log files, it removes the old log file with the smallest timestamp. num must be at least 2. The default number of log files is 10.