Good Question. In the past with limited mobility the planet's human population was probably broken up into several smaller effective populations with limited gene flow between them. E.g. pre Columbus there was not much flow between America and europe. Although the world is still somewhat separated people are much more mobile these days and there is more gene flow possible. Just think about the recent mass migrations from the Middle east and Northern Africa to Europe. I think the effective population today would be the total world population of 7 billion people.