There are various ways to restrict root access to varying degrees, but not by just the number of SSH logins (and even that would be easily circumventable with, for example, su).
why wouldn't you create a group 'sshusers', put your user with user id in that group and 'PermitRootLogin no' and 'AllowGroups sshusers' in openssh-server config? that'd be a kind of security. using pubkeys instead of password authentication is a good choice also.
I'm not convinced that you tried either suggestion, because both suggestions should work.
I go back to my original question: "why would you want to do that?" And I can't think of a single reason. If you were a capable Admin you would not have to ask this question.