It's an interesting question, but I think it's incomplete, because you talk about people's wealth, but you don't say anything about how much you need these people to invest in stock market.
Let's assume we want to calculate how many of these people want to invest $10.000 in stock market.
Also clarify with the interviewer if they'd like you to count households or individuals, it can depend on the country.
1) Let's take individuals. So you first identify people whose net worth is about 250k-2Million.
2) Then you find out how many of them CAN invest these 10.000, meaning they have this amount of liquid money.
3) How many of them WILL or WANT to invest into stock market:
- Stock market is medium to high risk investment, so you can filter out those customers who don't want to take risks normally (ex, elder people).
- Out of the people who are left, take out those who has already been exposed to high risk investments.
4) The number of people left ultimately are those who will be willing to invest into stock market.
Hope it was helpful,