Hmph. Well, Im almost certain there were 6 in 1st Edition.The 2nd edition Monstrous Manual says:
Habitat/Society: Balors exist for one purpose only: to wage the Blood War. They are driven by the strongest primal desire to fight and will command whole legions of beings into battle.
Balors also recognize a subtle but important aspect of the War: the prime material plane. They will often make pacts with mortals to perform services for the purpose of gaining power in the prime material plane. Balors know that the more followers or allies they can gain in the prime material, the more power they will have there.
There are at least 24 balors known to exist. The creatures do not inhabit any particular area of the Abyss, but wander about forming armies for battle with the baatezu (q.v.).
Ill continue searching online, as most of my books are on long term loan.