SLC-500 CPU's can address up to 30 slots of I/O, which can be in up to three chassis connected with bus cables.
It's possible to connect more slots than can be addressed (like with 13-slot chassis) and it's also possible to use more modules than can be addressed in memory (like analog or specialty modules). The limit as I recall is 4096 Words of memory.
That's off the top of my head... the User Manual has the specifics.