Since this is an interviewer-led case, the interviewer should guide the candidate through the interview.
There are several ways to estimate the number of gas stations. Their feasibility may be influenced by the amount of information the interviewer decides to give the interviewee. (In this case, there is not much information.)
Generally, an estimation method should use values that can be easily estimated and sanity-checked (verified using another approach).
How could you estimate the number of gas stations in Paris?
Which approach is best to calculate the number of gas stations in Paris?
How many gas stations are there in Paris?
Suppose your friend wants to open a gas station in Paris. What aspects should he consider?
If the interviewee solves the case very quickly, you can come up with more challenging questions to ask them.