How do I make a wave spawn timer?

I assume you want them to spawn in the same place, if so do this:

What you should do is make a folder in ServerStorage called NPC, in that folder (If you have more than 1 NPC type) put Folders called your NPCs Types (e.g Zombies), and in that folder put your NPCs (position them in the workspace first!). Then, in a script (probably in workspace), put this code:

while true do
    wait(time)
    for _, zombie in ipairs (game.ServerStorage.NPC.Zombies) do
        zombie:Clone().Parent = game.Workspace
    end
end

What it does is makes an infinite loop, and in that loop waits a certain amount of time, then uses a for loop to clone each zombie and place it in the workspace.