The lowest order bit of the first byte must be 0 for a station MAC. If it is 1 then it is a multicast MAC which can never be a source MAC.

A MAC can also never be all zeros.

So I think it works out to something like (2^47)-1. I'm probably forgetting some other specifics so it might even be lower than that.

There's a bunch of incorrect information on the link in parent. A simple example is that 01:00:5e:00:00:01 is NOT a valid source MAC address.