Who all, what all: những ai, những gì

Put những before a question word and it asks for a whole list: Những ai đi? (Who all is going?) Em mua những gì? (What all did you buy?) The person asking expects several answers, not one. Những is the plural word from những người này; here it does the same job for ai (who) and gì (what). Often có comes first: Tối nay có những ai? (Who's coming tonight?)

The same pairs also work without a question, meaning "whoever, whatever, everything that": Những ai muốn đi thì đăng ký (whoever wants to go, sign up). Tôi nhớ hết những gì cô dạy (I remember everything you taught us). This use is common in writing too: in notices, Những ai có nhu cầu … (those who wish to …), and in the news, những gì đã xảy ra (what happened).

For things you can count, những cái nào, or những + noun + nào, asks "which ones": Chị lấy những cái nào? (Which ones are you taking?)

You don't need những for every question about several people. Ai đi? is fine when you don't know if it's one person or more. Add những when you expect a list and want all of it.

One trap: mấy ai is not "who all" but "hardly anyone": Mấy ai biết chuyện này (hardly anyone knows about this).