"One" and "ones" can be used to substitute for a noun that was just mentioned.

Look at this example from the dialogue:

Dr. Li: It looks like the filling is loose.
Tina: Isn't that the same one you replaced last year?

Tina says "one" rather than repeating the noun (filling). She could have also said: "Isn't that the same filling you replaced last year?"

Other examples with "one":
▶ "Do you like the red or the blue jacket?" - "I like the red one."
▶ "I don't have a tie to wear to dinner." - "Let me lend you one."
▶ "Which ring do you like?" - "I like the one on the top shelf."
▶ "I'd like a chocolate chip cookie." - "We have two different kinds, one with nuts and the other without nuts. Which one do you want?"

When referring to a plural noun, use "ones."
Examples:
▶ "Those pastries look delicious!" - "Which ones?"
▶ "Please water the plants." - "Which ones?" - "The ones in the kitchen."

Look at another example from the dialogue:
Tina: Also, one of my top right teeth is a little sensitive.
Dr. Li: Which one?

You may wonder why the dentist doesn't say, "Which ones?'" since "teeth" are mentioned. In this case, however, we know Tina is talking about one of her teeth.