The restrictions to the uploaded item count per catalog is 500k items. If the supplier wants to upload more than this amount, you have the option to generate an additional catalog container. The supplier can then split up their items accordingly.
If the suppliercatalog does not contain a lot of special features like docfields, multiple article prices with relations, mimes, it is possible to upload a container with up to 600k items. However, it is generally not recommended to exceed the limitation of 500k items.