How many people could fit on the Moon?
It depends how tightly you pack them in.
The Moon has a surface area of about 38 million square kilometers (14.6 million square miles). If everyone on the Moon was standing or sitting, taking up about 1 square metre each, you'd fit 38 billion people.
Of course that's not very practical. People need a lot more space in order to survive. If we assume that each person needs at least 10 m2, then you could fit 3.8 billion people.
Here's another way to imagine the situation: The Moon has roughly 4.8 times the surface area of mainland USA. Therefore, at the same population density as mainland USA, you'd fit about 1.4 billion people on the Moon.