Igor good day,
in OneBox, there is no limit by the number of contacts, products, categories, processes - all the limits are connected to the resources of the server itself (as a rule, you can change it)
Our template for an online store can also be widened, in this case, I admit that you want to display goods from subcategories - enter the names of these subcategories. Tse two okremі nalashtuvannya. І two variants of vision:
1) in the main category you have one look here /admin/shop/grid/category/ - and all subcategories of it are inherited
2) the appearance of the skin specific category and subcategory can be adjusted by substituting the category ID: /admin/shop/grid/category/ID/ , the block itself is "Product category block"
https://www.screencast.com/t/MKPjqbeHDzCB