Steam Spy is a service that reports the number of owners of games on Steam using their API and a little modelling/guesswork.
Entrepreneur Math is when you make up numbers based on unrelated or partial data to prove to yourself how successful you’re going to be. For example: “There are 80,000 searches a month for ‘Cats Wearing Heels’. If my website, catswearingheels.com, gets 10% of those searches, then that’s 8,000 visitors a month. If 1% of those then go on to buy two pairs of my designer cat heels, then I can easily make $4,000 a month”.