There are actually 13 Unknowns. 6 only appear in Kingdom Hearts:Chain of memories though.

There are Axel, Larxene, Lexius, Zexion, Vexen and Marluxia.
But Currently it seems that Vexen is the highest rank (4) out of all of them. Marluxia is the leader but he is the lowest rank (12). It does not make sence. Larxene is 11, Axel is 6, Lexius is 6.