A classical PC master boot record does not actually have 512 byte for code as it also contains the partition table and a signature, you have 446 bytes for code.

A classical PC master boot record does not actually have 512 byte for code as it also contains the partition table and a signature, you have 446 bytes for code. Not sure what exactly the BIOS validates, you might be able to get away with an invalid partition table. In general there is not really any limit unless you want to be compatible with something existing, you can define whatever disk layout you like. At worst you will have to load additional sectors yourself because the BIOS has no clue where you put them. I no longer remember what a floppy boot sector looks like, how much room you have there.