The game has 23 on disc, 24 with Metal Sonic. As is, with the newly revealed character, we have one slot remaining. S0L spoke often about a big fanservice character that has yet to be revealed, though I don't think he has ever referred to it as a "final unlock". Rather, S0L keeps referencing 29 characters in total, and slyly hints that we will not know of all the characters until, perhaps, the end of the year. My guess is DLC, with SEGA holding the key. Thus why S0L knows more is to come, but can't say when.
As of now we have:
23 on disc
1 bonus racer
2 console exclusive avatars (Mii and NXE Avatar)
1 more guest, possibly a PC exclusive
That leaves us with 2 more mystery characters (in addition to the one "ultimate" racer on disc), and add in the third guest and we have that "2/3" you are referring to. My guess is 360/PS3/WiiU will see two more SEGA racers in the coming months as DLC, and PC will see a third guest and possibly the 2 more racers when they hit consoles.