You can return as many items as you like at once, providing they are packaged in their original packaging and together fit within one larger parcel, and that the return is requested at the same time. This is because your returns label is valid for only one parcel. Returns will not be accepted for multiple parcels with printouts of the same label attached.
If you do not have a large enough parcel to fit multiple items, don’t worry. You can simply request to return each item separately and print a label per product you’d like to return.