How many (a small number): mấy before the noun

Mấy before a noun asks how many when you expect a small number: Em có mấy anh chị em? How many brothers and sisters do you have? There's no fixed cut-off at ten or at twenty; what counts is your guess. If the answer is probably under ten or so, mấy is natural. If it could be anything, use bao nhiêu.

Mấy stands where the number will go, so the answer just swaps in a number: Hai người. Like a number, it takes the counting word the noun needs: mấy con mèo (how many cats), mấy trái dừa (how many coconuts; in Hà Nội mấy quả dừa). Nouns that take no counting word after a number take none here either: mấy người, mấy ngày.

It's for things you count, and for units like ký (kilo) or tiếng (hour): Chị lấy mấy ký? For money, prices and amounts that could be large, use bao nhiêu. For an adult's age, bao nhiêu tuổi is the polite choice; mấy tuổi is mostly for children.

After the noun, mấy asks which number instead: mấy lầu is how many floors, lầu mấy is which floor. And in a statement, with no question, mấy means "a few": Tôi ở đó mấy ngày. I stayed there a few days. Before people or things you can point to, mấy can also just mark more than one: mấy cái này (these ones), mấy bạn (you guys).