30-ish Black Court Elders. Thirteen is the number of Grey Council Members and the max number that can collaborate on a spell via a Circle, but it sometimes gets confused in discussion
I'm not sure I trust that 13 limit, either. Harry claimed it was so way back in
Storm Front, yes, but that might have been limited information on his part. It may well be the usual limit, but JB has at least implied that ways around it exist.
He once said that in theory, the White Council could take on Mab, if they had her Name, and all the Wizards in the Council combined their strength to do it. That at least implies that it's possible to make spell-combos and spell-circles with more than 13 members...or maybe you can combine such groups of 13 into larger groups of groups, or something.
But per JB's comment it's apparently theoretically possible for the whole Council to hit you at once. If you don't happen to be Mab or Titania or a Mother or an Archangel, I suspect that would hurt. A lot.