8 mil is for the five boroughs. That is, New York City.
May I see a source for your 22, please?
Well, get yourself in a helicopter above Times Square, then allow yourself the rough equivalent of what they consider Greater London, Ciudad de Mexico, or Los Angeles, roughly fifteen miles in all directions. You'd see where the population comes from.
New York City abuts the New Jersey border, so the entire community on the west shore of the Hudson River (Hoboken, Jersey City, even out to Newark) is really a feeder community to New York. There are subways (PATH trains) running out to Newark.
If you go in the other directions (East and North), there are dozens of communities that don't exist without New York. Basically all of Westchester County and all of Long Island up to The Hamptons and a little farther.
I once added them all, and you come up with a Metropolitan area of 22 to 25 million.
If you follow MacLuhan's old adage about a metropolis representing the extent of it's broadcast, then that whole area is essentially part of New York. There are TV and Radio stations situated in other incorporated cities/communities, but only for tax and cost reasons. They're there to service the metropolitan area.
I live in Hong Kong, which claims 8.5 million. The density here is unbelievable, but mostly due to much uninhabitable land. The density of traffic and people in New York, just to the eye, is much greater. There is no way that Hong Kong is the same size as New York. (At least not right now.... in ten/fifteen years as they fill in the area between Hong Kong and the old border, which will cease to exist according to the government in 2022, Hong Kong will be a metropolis of about 20/25 million also.)