I’m working on a two-zine game myself, so I’m inclined to allow it! 😁
I also don’t really have a hard limit in mind with regards to additional material. Whatever your game needs!
Of course, if you have hundreds of pages of additional material, someone might question if the game is really in the spirit of the jam, but, like, it’s not a competition or anything so go right ahead and do what feels right for your game! We’re just jamming, anyway.