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?
Demonstratives; one, ones