Need help with While loop, How do I decrease the number value when Humanoid's health is 0?

I have a script that spawns a random zombie within a folder onto one of the 6 random blocks within a folder in the workspace. I have it so every time a zombie spawns, the number value is goes up by one.

But how do I get t so when you a zombie dies the number value decreases by one. And in turn will let the while loop keep going and replace the dead zombie?

local NPCS = game.ReplicatedStorage.Enemies:GetChildren()
local number = 0

if number >= 0 then
        canspawn = true
        print("can spawn")
    end 


while  canspawn do
    local ChosenNPC = NPCS[math.random(1, #NPCS)]
    local ChosenSpawn = math.random(1, 6)
    print(ChosenNPC)
    print(ChosenSpawn)
    wait(5)
    if ChosenNPC then
    clone = ChosenNPC:Clone()
        number = number + 1     
        print(number)



        clone.HumanoidRootPart.Position = workspace.EnemySpawns:FindFirstChild("Spawn"..ChosenSpawn).Position
        clone.Parent = workspace.EnemySpawns.Spawned
        clone:MakeJoints()
        if number >= 5 then
            canspawn = false
            print("Stoping spawning")
        end 



    end

end