Your question is Energy Market Guesstimation. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How would you estimate the number of iPhone users in India?
Clarify whether the estimate should represent the installed base of people currently using an iPhone or annual iPhone sales. State your assumptions, build the estimate in logical segments, and explain how you would validate the result with available data.