something do you want

We use one (singular) and ones (plural) to avoid repeating a subject we've already mentioned. We use one and ones when it's clear what they refer to.

Info

Writing

Read the online ad. What items does Emilia want? Why does she want them? What kindof personality do you think she has?

In the question

below, one = a Canadian traffic light:

I'm interesting in buying a Canadian traffic light.Do you have one like this?