Source: http://www.proftpd.org/docs/howto/BCP.html
To set the number of concurrent connections for Proftp, we need to set the following configuration directives in the Proftp configuration file: MaxInstances, MaxClients, MaxClientsPerHost, MaxClientsPerUser, MaxHostsPerUser. These directive are set when Proftp is running in standalone mode
If Proftp is running in inet mode, then per_source and cps attributes need to be set in the xinetd configuration file