As the game clearly states the game is designed for 2-5 players. This is based on the game mechanic and resource level of the namesake kitten. Once you draw an exploding kitten and cannot defuse it, you are out and then the last remaining player winds. Since there are only 4 exploding kittens you can only have 5 players or there would be multiple winners.
Would it be possible to combine two whole game sets to allow for up to a maximum of 9 players (since there are now 8 exploding kittens)? Would this change cause any other game mechanics to break or be unfair?