I don't think there are quite that many. 7billion/20million (smallest plural 10 of million) get's us 350 people per "metro" this is assuming no one on the planet lives rurally (about half do). There is perhaps thousands of metros on the planet.

On a more serious note, realistically this number is much smaller for each person. After you take into account the ones you could legally live in, ones you can speak the language for, ones with good job prospects. A person in the USA is lucky to have tens of options. For people in smaller countries the options are likely to be under a dozen.