I've just done this in real life :)
This really depends on how perfect the interviewer wants it to be. Meaning that I can do it taking 3 mins… or 30 mins.
Number of chargers = Charging demand / (Charging capacity of a single charger* capacity utilization)
Now the tricky part is how to you estimate capacity. Because it can be done in “kms of power”, in Kilowatts, etc…. I would probably do it in “full charges” and assume a full charge takes 4h (considering slow and fast charger average*) and provides 500 kms.
*Please note that one could deep dive and consider the multiple types of charges according to use case (in transit, parking, etc… I am assuming that most of chargers will be in parking and will be slow)
Capacity of a charger = 16 working hours / 4h *365 = 4 charges / day * 365 = ~1400 charges per year
Capacity utilization = ~70%
Therefore used capacity = 1400*70% = ~1000 charges/year
Charging demand = number of electrical vehicles * average kms driven a year by an electric car / 500 * % of charges done outside home
1.000.000 * 10.000 / 500 * 40% = 80.000.000
[Please note that a better way to do this would segment the charging demand between families and professional users…]
So number of chargers = 80.000.000 / 1.000 = 80.000 chargers
The tough part in this exercise is that one can be wildly wrong and have to idea… and there is a ton of assumptions above that the acceptable range of answers is so high that it is very easy to be completely wrong. Now, remember that the objective here is to show your thought process. You don't need to know how long it takes to charge an Electric Vehicle, you don't need to have a strong sense of capacity utilization not how frequently do people charge outside their home. That is why you should validate (or ask for support when defining) your assumptions with the interviewer.
Hope this helps, and happy to get any feedback on my own answer.