Rebex File Server, our SFTP server library, doesn't currently have any built-in or configurable limitations on the maximum number of concurrent sessions.
In scenarios where it is sensible to limit this to a reasonable number (in order to prevent the server from running out of resources, for example), the recommended solution is to use FileServer object's Connecting and Disconnecting events (see here) to keep track of the number of active sessions, making it possible to refuse additional connection attempts in the Connecting event if there are already too many sessions.