How to restrict number of sftp sessions on a per user basis

Issue

• A valid user opens more and more sftp sessions, in the end the network stack overflow will kill all access to the machine, and other connections will not be accepted anymore.
• MaxSessions setting in sshd_config does not restrict number of individual sessions.
• maxlogins in /etc/security/limits.conf limits individual ssh sessions on user basis, but not number of sftp sessions on per user basis.