How do I set text based on how many children there are under the parent part?

Hello! I'm a beginner scripter so don't expect me to know a lot. In my game,there's a part where when you hover over it once, it spawns 1 part. I'm trying to make a textlabel where when a part spawns, it says 1 and goes up by 1 whenever 1 part is spawned. I put the parent of the parts that are going to spawn to part3. The goal for the game is in a certain amount of time, the player has to hover over the block repeatedly until it reaches 150 parts. Here's the code for the textlabel:

local text = game.StarterGui.ScreenGui.TextLabel.Text
text = "0"

repeat 
	if game.Workspace.part3.ChildAdded then
		local newtext = text + 1
		text = newtext
		wait(5)
	end
until text == "150"

I tried to do GetChildren() and a function but no luck. Any help would be appreciated!

You can do:

local textLab = game.StarterGui.ScreenGui.TextLabel
local PartNumber = 1
local Done = false
local Time = 0

game.Workspace.part3.DescendantAdded:Connect(function(Descendant)
if Descendant:IsA("BasePart") and PartNumber < 151 then
PartNumber = PartNumber + 1
textLab.Text = PartNumber
elseif PartNumber == 150 then
Done = true
end
       end
end)

repeat wait(1)
Time = Time + 1
until Done

I would just put the code that change the text inside the code that adds a new part.

function addPart()
	--the code you use to add part--
	thenewpart.Parent = part3
	text = text + 1 -- this is the thing that change the thing

This in theory should work.