Manufacturer Product Number

Hi Guys, I’m listing my product on Amazon, last time I skipped this and it was fine. But I’m curious, should I add one in?? I’ve got a single product, 2 different variants a 4 pack and a 6 pack. Should I include the MPN or just leave it blank?? This is for a private label item I’m ordering from a supplier overseas. What do you think? Also what number should I add if you think I should, or does it not matter?? Thanks in advance !!