There are a few sources you can use to get information on how many users have downloaded your mobile apps:
In your ATOM dashboard you have the possibility to see how many users have registered in your apps - this would correspond to the majority of users who have downloaded your mobile apps. You can get this statistic by downloading the csv or xlsv file in Customer section. Please note that you have to set the date range, it can be up to 1 year. Once the file is downloaded, you can filter by the operation system (iOS or Android) and use the formula to sum up the specific group.
Another way to see the user count would be in OneSignal - you can go to your OneSignal account dashboard and see the total subscription amount, example below:
If you wish to compare how many people have downloaded the app and how many have actually registered, you would want to get the app downloads statistics directly from App Store and Google Play.
NB! Please note that it is not possible to get these statistics if you have previously decided to publish your mobile apps on ATOM Developer accounts on App Store and Google Play.
If you have your own organizational developer accounts in both stores, then you have to do the following to retrieve the statistics. In order to see downloads count in App Store, it is important to have Apple Analytics, you can read more about it on their official website here. In order to see downloads count in Google Play, you have to go to Play Console and there to Statistics, you can see more information about it here.