When not being used as a numeral, 'one' substitutes for a noun. Which book do you want? I'll take the small one (ie. the small book). There are several books on the desk - which one (which book) is yours? Are you going to catch the express train? No, I am going to catch the one that goes through the local villages. What sort of car are you going to buy? I want one that is big enough to fit all the kids and my fishing gear in.

In this instance, the 'one' is just used to emphasize the object. It does not refer to a quantity. Like "I know this one place that has good food." The quantity of places is not relevant. It is just used in casual speech meaning "I know a place." You could also say "I know this place that has good food." Meaning is almost exact. The one does have a slight connotation meaning it's a place of many, but the many is not the meaning and is not the main purpose of the sentence.