Someone please refresh my memory for me.

to my knowledge there isnt a category limit or limit per category, just a total amount in gallery limit

There is no limit to numbers of items (presuming its less than the shop limit itself) within categories.

But I do recall a limit of 20 categories. The shop itself doesn't prevent you from making more than that, but when I did it all the items I added to categories above 20 went into the 20th category not the new ones. That said, its been many years since I tried that and it could be that happened with legacy shops rather than new shops. It may also have been early in the evolution of new shops, which did have a lot of changes made in the early days.

I recommend testing or submitting a ticket.