We use one (singular) and ones (plural) after:

an adjective:

Which is your car, the white one or the black one?

My tires are old. I need some new ones.

the:

My car is the one on the right, near the truck.

Which photos are you talking about - the ones you took in New York?

which in questions:

You can rent a car. Which one do you want?

Which ones are yours?