How many people want to buy your product?

It's a marketing thing of course.
To know ahead of time how many people want to buy your product? It's possible to know that.

By going to a professional marketing company of course, or doing it yourself.

How?

It's based on a 'trick'. It's name is counting fish in the pond.

How many fish swims in a large pond? Count fish in a hundreds part and multiply that with 100. You have the statistical value.

Same with counting the people who wants to buy your products.