Many of the items are hard to identify, and almost all look rather plain, almost symbolic. Simplicity is fine, and often a good thing, but I think most of these need more definition to feel like actual items rather than just general symbols/icons of broad kinds of items.

I think the brown pouch is the strongest one, it has a sense of volume and texture. The axe looks nice, although it also seems rather thick because of the shadows at the edges, looks more like a club than an axe (which would be thin and edge-shaped, to split things).

The shields(?) look completely flat, and could really use some shadows to show their curved forms.

Nitpick: The shadows on your reds are much darker than the shadows on everything else, so the teeth on the red key become lost. For some of these items, you'll have to put more work in than just palette-swapping them, since palette-swaps can cause these kinds of legibility problems.

I think the colours you have should be fine, you don't need to add more if you don't want to. The problems here are in construction, lighting, and texturing, not the colours.

That said, colour count isn't related to "quality". Small palettes can look good, but so can large ones. It's about how you use those colours, and the relationships of the colours within the palette. Use as many or as few colours as you need for the look you want.

Hi. Nice set. The palette is too pastel to my personal taste, and the metal in the objects doesn't look like it: the greys are too neutral and there's no lighting. The the key handle looks too thick. The steel ring would be much nicer with another lighter color, and the shape of the white line to depict glass in the potions looks a little odd.

Try adding some shadows to the sides of the shields to show their curvature. For the potion bottles, rounder highlights would probably look better.

Even though the items don't have to be to scale with one another, I think the rings will look more like rings (as opposed to headbands, shields, etc) if they're smaller and don't take up the entire box.

You could also orient them diagonally instead of making them vertical, them being vertical makes it look like they have to be that way, as if they're fixed to the floor.