how do u make things (e.g a zombie) spawn repetitively?

Just clone the model to your desired location. For example you can spawn an NPC at 0, 0, 0. To do this, insert this script below in ServerScriptService. But, since it's a model, you want to position it, you would do this: (Make sure to set the PrimaryPart of the NPC's Model to anything inside of it)

local NPC = game.Workspace.Zombie

while true do
    wait(10) --Wait 10 seconds to spawn in a new NPC
    local clone = NPC:Clone()
    clone.Parent = game.Workspace
    clone:SetPrimaryPartCFrame(CFrame.new(0,0,0))
end