All of them (plural): các

A Vietnamese noun doesn't change for the plural: sinh viên is one student or many, and usually the situation makes it clear. When you want to point to a whole group you both know about, put các in front: các sinh viên (the students, all of them), các món ở đây (the dishes here).

That's how các differs from những, which picks out some members of a group, usually with a description: những sinh viên giỏi (the students who do well). A sentence with các often adds đều (all) before the verb: Các phòng đều có máy lạnh.

For things in general, leave các out: "I like dogs" is simply Tôi thích chó. After a number there's no các either, because the number already shows how many: ba sinh viên.

You'll see các on signs and in writing, and hear it when someone speaks to a group: các bạn, các em. In relaxed Saigon speech, mấy often takes its place: mấy bạn, mấy cái này (these ones).