Why has my while loop stopped looping?

I did this a while back so I forgot how it worked, but my intended purpose was for the while loop to create parts until there is 20 parts in the game and stops. Then continues again when there is less than 20 parts in the game.

How can I restart the loop again?

local model = workspace.Model

while #model:GetChildren() <= 20 do
	local part = Instance.new("Part")
	local cd = Instance.new("ClickDetector", part)
	local xpos = math.random(-100,100)
	local height = 20
	local zpos = math.random(-100,100)
	local model = game.Workspace.Model
	local children = model:GetChildren()
		part.Parent = workspace.Model
		part.Position = Vector3.new(xpos,height,zpos)
		print(#children)
		wait(2)
		
cd.MouseClick:Connect(function(player)
	local score = player:WaitForChild("leaderstats"):WaitForChild("Score")
		score.Value = score.Value + 1
			part:Destroy()
				
			if score.Value > 10 then
				score.Value = score.Value + 1
			end
			
			if score.Value > 20 then
				score.Value = score.Value + 1
				
		end
	end)
end

the reason it is not looping is because it is only looping the amount of Children is inside model

here is the fix

local model = workspace.Model

while true do
	if #model:GetChildren() < 20 then
    local part = Instance.new("Part")
    local cd = Instance.new("ClickDetector", part)
    local xpos = math.random(-100,100)
    local height = 20
    local zpos = math.random(-100,100)
    local model = game.Workspace.Model
    local children = model:GetChildren()
        part.Parent = workspace.Model
        part.Position = Vector3.new(xpos,height,zpos)
        print(#children)
        wait(2)

cd.MouseClick:Connect(function(player)
    local score = player:WaitForChild("leaderstats"):WaitForChild("Score")
        score.Value = score.Value + 1
            part:Destroy()

            if score.Value > 10 then
                score.Value = score.Value + 1
            end

            if score.Value > 20 then
                score.Value = score.Value + 1

        end
    end)
end

This code will accomplish what you are hoping for (making sure workspace always has at least 20 parts). Hopefully you can visualize how to add specific things to these parts on your own.

function PartAdder()
	local count = 0
	
	for _,v in pairs(workspace:GetChildren()) do
		if v:IsA("Part") then
			count = count + 1
		end
	end
	
	if count < 20 then
		for _ = 1,20-count do
			local inst = Instance.new("Part")
			inst.Parent = workspace
		end
	end
end

workspace.ChildRemoved:Connect(PartAdder)

PartAdder()

Essentially what we're doing is hooking a function which will make sure workspace has at least 20 parts to the inherited ChildRemoved event of workspace. This way, whenver a child is removed, it will check to make sure that 20 parts still exist within workspace.

We then run the function once to ensure that workspace has at least 20 parts when the game starts up.

If you have questions feel free to ask, I may not have explained this super well -- but it does work how you want.

Alright, so what it seems like what you wanted was to have 20 parts created and then let the player click all of them until they were gone, and then spawn a new 20 parts, so I basically gut your entire code and rewrote it with a simpler recursive design:

local m = game.Workspace.Model

function createParts(model)
	while #model:GetChildren() <= 20 do
    	local part = Instance.new("Part")
    	local cd = Instance.new("ClickDetector", part)
    	local xpos = math.random(-100,100)
    	local height = 20
    	local zpos = math.random(-100,100)
    	local children = model:GetChildren()
    	part.Parent = workspace.Model
    	part.Position = Vector3.new(xpos,height,zpos)
    	print(#children)
    	wait(2)
	end
	killParts(model)
end

function killParts(model)
	while #model:GetChildren() > 0 do
		model.Part.ClickDetector.MouseClick:Connect(function(player)
			local score = player:WaitForChild("leaderstats"):WaitForChild("Score")
        	score.Value = score.Value + 1
            model.part:Destroy()

            if score.Value > 10 then
                score.Value = score.Value + 1
            end

            if score.Value > 20 then
                score.Value = score.Value + 1
        	end
		end)
	end
	createParts()
end

createParts(m)