How to use the PlayerNumbering script?

Hi guys, I'm still learning PUN 2 and facing a problem I can't solve.

I've tried to use the PlayerNumbring script but it always gives me an error that I don't know how to solve

public static int GetPlayerNumber(this Player player)

It gives me an error in SpawnPlayers :

public class SpawnPlayers : MonoBehaviourPunCallbacks

It says "Extension method must be defined in a non-generic static class"

Can any one help?