Work out ssh session limit instantly with clear inputs, formula shown and shareable results.
Each SSH session forks a privilege-separated process pair plus the user's shell, costing tens of megabytes. MaxSessions limits multiplexed channels within one connection, while MaxStartups limits unauthenticated connections in progress — the latter is what throttles a fleet of clients reconnecting simultaneously after a network event, producing confusing intermittent failures.
Session capacity
sessions = users x sessions per user; memory = sessions x per-session memory; MaxSessions ~ sessions per user x 1.5
MaxStartups randomly drops connection attempts once the unauthenticated backlog exceeds its start threshold. Raising it, or using connection multiplexing, avoids the problem.
The number of open channels — shells, port forwards, SFTP subsystems — multiplexed over one TCP connection. It is not the number of separate logins.