We use one (singular) and ones (plural):
- after an adjective:
See those two girls? Helen is the tall one and Jane is the short one.
Which is your car, the red one or the blue one?
My trousers are torn. I need some new ones.
- after the:
See those two girls? Helen is the one on the left.
Let's look at the photographs – the ones you took in Paris.
after which in questions:
You can borrow a book. Which one do you want?
Which ones are yours?