How many bytes does a would-be intruder have to get exactly right?

You can of course combine several methods (and piss off your users), or use two factor authentication (OpenSSH supports several schemes).

Authentication method	Number of bytes
Password	Password length (varies, how long is yours?)
Alternate Port	Port number (2 bytes, it's a 16 bit value, remember)
Port Knocking	Number of ports in sequence * 2 (still a 16 bit value)
Single Packet Authentication	2 bytes (the port) plus Max 1440 (IPv4/Ethernet) or 1220 (IPv6/Ethernet)
Key Only	Number of bytes in key (depending on key strength, up to several kB)