This is the quickest way to limit the number of connection to your SSH server with iptables.
sudo /sbin/iptables -A INPUT -p tcp --syn --dport 22 -m connlimit --connlimit-above 5 -j REJECT
This will only allow up to 5 concurrent connections to the SSH server, subsequent connections will be rejected by iptables, thus this can thwarts Brute-force attempts to your server.
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