We run sshd in a socket activated per session mode by default so some sshd_config settings like port or number of connections won't be applicable since sshd isn't responsible for whole system settings like that. Instead the sshd.socket unit can be customized to manage such things. For example you should get what you want by writing the following to /etc/systemd/system/sshd.socket.d/limit.conf
[Socket]
MaxConnections=1
See https://www.freedesktop.org/software/systemd/man/systemd.socket.html