How would you use table.sort to organize a table of strings with differing end numbers?

I'm making a custom backpack, and it was going surprisingly smoothly up until the point that I needed to add items into certain slots. When an item is added into the player's backpack, my script runs through my custom backpack slots and checks if the ImageLabel is clear or not (if it's not clear, an item already occupies it). I've named my item slots ItemBox1, ItemBox2, and so forth. How can I make the script sort through the item boxes linearly, checking first ItemBox1, then ItemBox2, etc.? I imagine that I'd have to use table.sort, and if that's the case, can I have an example of how the function would be used?

backpack.ChildAdded:connect(function(child)
	if child.ClassName == "Tool" then
		table.insert(items, child)
		print("item added")
		for _, v in pairs(backpack:GetChildren()) do
			table.insert(items, v)
			for _, w in pairs(items) do
				image = w.TextureId
				tip = w.ToolTip
				for _, box in pairs(backpackitems:GetChildren()) do
					if box.ImageLabel.Image == "" then
						print("found a box")
						box.ImageLabel.Image = w.TextureId
						return
					end
				end
			end
			print("bag loaded")
		end
	end
end)