http://www.die.net/doc/linux/man/man8/pure-ftpd.8.html
-c clients
Allow a maximum of clients to be connected. clients must be at least 1, and if you combine it with -p it will be forced down to half the number of ports specified by -p. If more than clients are connected, new clients are rejected at once, even clients wishing to upload, or to log in as normal users. Therefore, it is advisable to use -m as primary overload protection. The default value is 50.