No những / các after a number, no plural word when the context shows it: ba con chó

A Vietnamese noun has one form for one and for many: sách is a book or books. English marks almost every plural; Vietnamese adds a plural word only when it says something extra.

After a number, nothing more is needed, because the number already says how many: ba con chó (three dogs), with no các or những. The same goes after mấy and vài (a few): mấy người, vài cuốn sách.

Without a number, the bare noun is plural whenever the situation makes it so: Ngoài đường đông xe quá. There's so much traffic out there. Em thích xoài. I like mangoes. Nhà em có khách can be one guest or several.

So when do you add a plural word? Các is for all of a group you both know about: các phòng ở đây (the rooms here). Những picks out some of them, usually with a description: những ngày mưa (rainy days). If neither fits, leave the noun bare.

Keep những away from numbers, too. Những ba tiếng doesn't simply mean "three hours" but "a whole three hours", an amount that surprised you.