There are roughly around 167 Arbiters. The one in Halo Wars is the 16th or 17th Arbiter, while the one in Halo 3 is the 166th or 167th Arbiter in the games.
in halo 2 the golden arbiter is still a Arbiter before he gets his armor. Actually, Thel 'Vadamee only became the Arbiter after he accepted the task of assassinating Sesa 'Refumee. Until then he was a Supreme Commander, then a heretic after the opening cutscene.