Ravidrath's said they're aiming for a total of 27 playable Incarnations. (Plus currently 7 playable guests and one IGG donated character, but we'll ignore those)
So far 20 incarnations have been revealed on the campaign page,
with 6 more shown in trailers.
That means there's one more unrevealed playable, if they stick to the 27 plan anyway.
At $1,250,000 they should run out of payable incarnations and switch over to the NPCs.
There's a host of NPC incarnations that will live in Ajna's Inner Realm
acting as quest givers and lore keepers, etc. God knows how many of those there are!