How do I make the script keep on the script to keep on checking the numbers?

Well you can create a loop to change that text

Let's use a while loop for this

while true do wait()

end

it's important to remember for a while loop to place a wait() somewhere in there so it doesnt crash your game

Next inside the loop let's change the Text of Text to what the value of the cookies variable is

while true do wait()
	Text.Text = plr.PlayerGui["Cookies/CPS"].Frame.Cookies.text
end

Take note of how I went to plr.PlayerGui instead of game.StarterGui, also its best if you make the wait a bit longer to not add to the lag, although it is little in this case.

Unrecommended version, Scroll down

local plr = game.Players.LocalPlayer

Humanoid = game.Workspace:WaitForChild(plr.Name)
Part = Instance.new("BillboardGui",Humanoid.Head)
Text = Instance.new("TextLabel",Part)
Part.ExtentsOffset = Vector3.new(0,3,0)
Part.Size = UDim2.new(1,1,1,1)
Text.Size = UDim2.new(5,0,1,1)
Text.TextScaled = true
Text.Font = "SourceSansLight"
Text.BackgroundTransparency = 1
Text.Position = UDim2.new(-2,0,-0.5,0)
Part.AlwaysOnTop = true
while true do wait()
	Text.Text = plr.PlayerGui["Cookies/CPS"].Frame.Cookies.text
end

If you want a more efficient way to change the text we can create a changed event function to only change the text of text when the text of that gui changes

plr.PlayerGui["Cookies/CPS"].Frame.Cookies.Changed:connect(function(c)
	if c=="Text" then
		Text.Text=plr.PlayerGui["Cookies/CPS"].Frame.Cookies.Text
	end
end)

c is the property that was changed

Good one

local plr = game.Players.LocalPlayer

Humanoid = game.Workspace:WaitForChild(plr.Name)
Part = Instance.new("BillboardGui",Humanoid.Head)
Text = Instance.new("TextLabel",Part)
Part.ExtentsOffset = Vector3.new(0,3,0)
Part.Size = UDim2.new(1,1,1,1)
Text.Size = UDim2.new(5,0,1,1)
Text.TextScaled = true
Text.Font = "SourceSansLight"
Text.BackgroundTransparency = 1
Text.Position = UDim2.new(-2,0,-0.5,0)
Part.AlwaysOnTop = true
plr.PlayerGui["Cookies/CPS"].Frame.Cookies.Changed:connect(function(c)
	if c=="Text" then
		Text.Text=plr.PlayerGui["Cookies/CPS"].Frame.Cookies.Text
	end
end)