There are far more than 2 million books in KU. They stopped disclosing the exact number years ago but they hit the first million in 2016, and the pace has picked up exponentially since then. I've seen books with ranks close to 18 million. If you were writing for exposure, it would be fine-- most hobbyists are writing for exposure. But you're not getting exposed if you don't buy ads/promotions which means the program is best suited to professionals.