how do i create a gui that counts how many players are in one place?

I want to make a GUI that will count how many players are sitting in a truck for my Story game's lobby. When I step on a button, it will put me in the truck but I don't know how to make the number larger on the SurfaceGUI I made according to how many people are sitting in the truck. Here is what I have done so far

--Variables
local leftValue = game.Workspace.LeftRoad.Truck.Value --To see if the truck exists in the Workspace
local CounterLeft = game.Workspace.Counters.LeftCounter.SurfaceGui.Count.Text
local LeftCountVal = game.Workspace.LeftRoad.Count.Value
local buttonLeft = game.Workspace.Entries.LeftEnter --Button when touched, makes you enter the truck

--Script to count players in the truck
if leftValue == true then
	buttonLeft.Touched:Connect(function(plr)
		if plr.Parent:FindFirstChild("Humanoid") then
			LeftCountVal = LeftCountVal +1
			wait(0.5)
			CounterLeft = LeftCountVal
		end
	end)
end

So I want the text on the SurfaceGui to be the same as the text on the IntValue (Which is the variable "LeftCountVal") some help would be greatly appreciated!