How do I disable a script after an amount of models have been detected in the workspace?

I have a script that clones random zombie models in a random position and into the workspace.

--variables
topLeft = Vector3.new(-85.3, 0.5, -30.9) --Top left corner of the 2d region
bottomRight = Vector3.new(-43.2, 0.5, 6.9) --Bottom right corner of the 2d region
az = game.ReplicatedStorage.zombiemodels.A
bz = game.ReplicatedStorage.zombiemodels.B
cz = game.ReplicatedStorage.zombiemodels.C
dz = game.ReplicatedStorage.zombiemodels.D
ez = game.ReplicatedStorage.zombiemodels.E
fz = game.ReplicatedStorage.zombiemodels.F
gz = game.ReplicatedStorage.zombiemodels.G

singletonRandom = Random.new(tick())

--Function for the random number

function GetRandom(Min,Max)
	return singletonRandom:NextNumber(Min,Max)
end

--Making the cloning items

PlayingFieldHeight = 15
while true do
 local phrases = { az, bz, cz, dz, ez, fz, gz }
 print ( phrases[ math.random( #phrases ) ] )
 model = (phrases[math.random(#phrases)]):clone()
 model.Parent = game.Workspace
 model.PrimaryPart = model.HumanoidRootPart
 model:SetPrimaryPartCFrame(CFrame.new(GetRandom(topLeft.X,bottomRight.X),PlayingFieldHeight,GetRandom(topLeft.Z, bottomRight.Z)))
 wait(2)
end

I am very very new to scripting so please cut me some slack here, but I need to find out how I can count how many models are in the workspace with the corresponding names "A, B, C, D, E, F, G" and then disable the script when the number has reached the threshold I give it. Any help on how I can do this? I have no idea.

I don't understand your script but I'll just do what the question asked

for i,v in pairs(workspace:GetChildren()) do
	if v:IsA("Model") then
		local number = #v
		if number >= 5 then -- If there are more than 5 models
			script.Disabled = true
		end
	end
end

local s = --Script here
while true do
    wait()
    if tonumber[game.Workspace:GetChildren()] >= 20 then
        s.Disabled = true
    end
end

That is what I would do.