There is a public ranking of apps ranked by downloads in the app store. If you know how many downloads correspond to what position in the chart (e.g. number 100 makes ~1000 downloads), you can fit a log regression to determine the number of downloads given the ranking in the category. I am oversimplifying because you also need to take into account seasonality and time trends (e.g. the app store is always expanding), but overall fits are very very precise (unless you get near the top positions, which are those for which they have more problems in estimating). Ah, and if you wonder where the data comes from, App Annie and Sensor Tower get data for downloads to fit their models directly from tens of thousands of developers that share it with them in exchange for free analytics,