How do I allow people to have only a limited amount of inventory space?

What I would do:

Make a numbervalue and name it maxinventoryspace make a numbervalue and name it currentinventoryspace

local MaxInv = script.Parent.maxinventoryspace -- Or where ever you put the number value
local CurInv = script.Parent.currentinventoryspace -- where ever you put it
local InvFullText = script.Parent.InventoryFull -- The textlabel that says inventory full
local Inventory = script.Parent.Inventory -- Where the inventory is

function CheckInv()
	if CurInv.Value == MaxInv.Value then -- Checks If Player Can Have Item
		return false -- Returns false which will indicate you cannot have another item
	elseif CurInv.Value < MaxInv.Value then -- Checks If Player Can Have Item
		return true -- Returns true which will indicate you can have another item
	end
end

Inventory.ChildAdded:Connect(function(Child)
	if CheckInv() then -- If CheckInv returns true
		CurInv.Value = CurInv.Value + 1
		-- Do what you need to do here
	elseif not CheckInv() then -- If CheckInv returns false
		Child:Destroy()
		InvFullText.Visible = true
		wait(2)
		InvFullText.Visible = false
	end
end