"00:14:24.755 Script timeout: exhausted allowed execution time - Server - ZombieScript:9"

-- It seems like the script is running into an infinite loop causing it to exceed the allowed execution time. To avoid this, you can add a wait() function inside the while loop to pause the script execution for a certain amount of time. Here's how you can modify the script:

local spawns = script.Parent.Spawns
local zombies = script.Parent.ZombieInstances

local max_zombies = 50

local wave = 0
local num_zombies = 3

while true do
    for _, player in pairs(game:GetService("Players"):GetPlayers()) do
        if player:FindFirstChild("PlayerGui") then
            player.PlayerGui.IntermissionAndWave.Wave.Text = ''..wave
        end
    end
    if #zombies:GetChildren() == 0 then
        script.NewWave:Play()
        wave  = wave + 1
        if num_zombies <= max_zombies then
            num_zombies = num_zombies * wave
        end
        for _, spwn in pairs(spawns:GetChildren()) do
            if spwn:IsA('BasePart') then
                for i = 1, num_zombies do
                    local zombieCopy = game.ReplicatedStorage.Zombies['Furry']:Clone()
                    zombieCopy.Parent = zombies
                    zombieCopy.HumanoidRootPart.CFrame = CFrame.new(spwn.Position + Vector3.new(0,3,0))
                end
            end
        end
    end
    wait(1) -- add this line to pause the script for 1 second
end