It shouldn't be possible to have more than 6 hand tokens on the board at the same time:
- The "Forbidden Alchemy" expansion only contains 6 crawling hand tokens
- The rules for placing them explicitly say
Investigator Guide wrote:
Each time a corpse is placed on the board (including during setup) place 2 crawling hand tokens (if available) in the same space.
That bold part makes it pretty obvious that the keeper isn't supposed to place more hand tokens on the board than that limited amount of 6.