Ssh – way to find out how many instances of the root user are logged into a Linux machine

System accounts should already be locked, by setting their password hash to an invalid one (I see x and ! in my /etc/shadow). The only ways to get into their accounts without a password (as far as I know) are ssh with keypair auth (impossible unless someone with write permissions in their home directory put a public key in their ~/.ssh/authorized_keys), you can use Alex's AllowUsers configuration to prevent that remote possibility; and using su as root to get a shell as them. This possibility is impossible to prevent if anyone has access to root power. But if they do have root permissions, suing to a daemon account is the last thing you need to worry about.

Many utilities will show the user's IP address, including who and w and last -ai.

Of course, if there is any form of redirection taking place, you may be looking at the machine that did the redirection.