one possible approach (top down approach - you can also go bottom up).
First clarify the perimeter of the market sizing e.g.
- Only privately-owned cars vs. taxis, professional vehicle included
1) Estimate population of the city in question
2) Estimate the number of families in the city using an hypothesis about the avg. number of people per family in the relevant country (is car-sharing between family members is significant in the country at hand - behaviours can change a lot across the world)
3) Now use an hypothesis about the % of families who own a car, two cars, three cars etc..
4) Then sum up the # of cars that you obtain
First level approach, can be further "deep-dived", if need be.
Hope it helps.