What does the number is the square brackets signify? I thought it meant port but the port number in the message is different from that number.

Nov 26 09:15:19 machine sshd[21221]: Postponed keyboard-interactive for wagner from ::ffff:205.150.86.5 port 43106 ssh2

should be the process number of the sshd process. check with "ps -ef | grep - ssh" or "ps -ax ..." if you are on bsd unix.