Hm I don't know if there is an actual cap of items in the character backpack. There is a cap for the ninja tool backpack and that's based on number of occupied slots (though items in ninja tool backpack don't stack anyway so). But like I have 12k+ dangos for instance so I would argue either the cap is really high, there is no cap, or it's based on number of occupied slots per backpack tab. I don't think there's an occupied slot cap though as I've never gotten a backpack full message ever. Each slot can hold a maximum of 9999 units by the way. Yay dangos.