I think the I've seen ones up to 10, I'd guess the largest possible would be 12??
I mean... technically it can be infinite if you just have ridiculosity turn it into a mystery. ¯\_(ツ)_/¯
I doubt that there are dice bigger than 12 since they wouldn't be able to be solved even with 4 friendships. Still this makes me want to search the game files for 9+ dice images
