Totally agree with Vlad - it's not at all about the true number, as long as your result is within the realm of possibility. If you say there are 7 or 70,000 gas stations in Paris I would question your judgment. But saying 120 or 300 and basing that on a solid set of assumptions is totally fine.

If I was the interviewee, with this specific case, I would actually try to steer the conversation into a different direction - how the number is changing due to the changes in urban transport (public transport, ride sharing, e-mobility...). Because that is a strategy discussion. Trained monkeys can count gas stations, and elementary school children can do the math for your assumptions. But only smart people can have a thoughtful strategy discussion. If you're face to face with the right interviewer, that might score you some points. If he just wants you to do basic math, then you can still go back to doing that...

BTW: Paris, in particular, is tricky, because the 2.2 Million refer to Paris "inter muros", so the actual city. The agglomeration is of course vastly larger, so much of the supply for the city might just be outside of the walls of the city (cheaper real estate, lower taxes etc.). Just like you don't have that many gas stations in Manhattan or the City of London...

(editiert)