We have a full team working on this for about 7 years. Constant improvements to our algorithms, new challenges, new technologies. A ton of other functionalities besides counting (e.g. forms, reports, integration). A lot of work on the backend, UX. Also, a lot of sales and marketing involved.
The key that allowed them to be a billion dollar business was marketing, hype, strategy all things that could apply to any product technical or not.
If you want to make something similiar you could probably get to 80% really quickly. Putting together a product with a reputation is a different event with timing luck and risk. Building that 80% is low risk but high personal reward.
Has anyone built something to read and identify palms or lines in the hands?