Isn’t that the point??
It moved. The Knight (bless its little quirkiness) couldn’t possibly have been checking any of the eight target Kings on the previous move while the Queen would have to have been checking at least two (alone the line that it moved). So all you need to get nine Kings in check is for the Knight move to discover an attack on the ninth King. I’ll leave the precise positioning as an exercise.
As long as we are looking at a variant allowing multiple kings, we may as well look at the existing variant where capturing a piece allows assuming the capabilities of that piece (until it actually makes a move that is only possible by using those capabilities). So white captured black’s queen with a pawn and black can capture back with a knight.
Tom’s alternate, 9-king check could actually become 11 if the white pawn captured a black knight: checking 8 kings using the knight’s threat capability; checking two more kings using a normal pawn threat; and uncovering a threat on another king that can capture the pawn/knight.
For that matter, a knight that had previously captured a pawn and used that capability to take a queen could check 8 kings using the capabilities of a queen, 8 more kings using the capabilities of a knight (since it didn’t use a knight move to make the capture) and uncover an attack on one more king that can escape by capturing the little monster (well, I guess that last, and apparently 17th, one would actually have to be one of the first 16 to make the capture of the monster).
PS are we limited to two-dimensional chess or can we extend into additional dimensions?
It’s never been clear to me what the right way to extend chess to 3D (or higher dimensions) is. I used to play 4 dimensional tic-tac-toe with some of the other CS students in college, we tried playing 5 dimensions, but with 7776 possible places to move (the board is 6x6x6x6x6), we ran out of time in the school year before we finished a game.