Google Play and the App Store do not share to the public how many players have pre-registered to play Diablo Immortal; but there are other platforms out there that do share the data.
It is this way how we can more or less gauge how popular the mobile game is to fans in certain areas of the globe.
Google Play isn't pre-installed in most Android devices in Asia.
TapTap is a mobile client used as a portal to download games and apps; and its mostly targeted at Chinese, Japanese, and Korean users.
Tap Tap shares that 1,043,448 users have pre-registered to play Diablo Immortal.
Founded in 2014, APK Pure is a portal to download Android apps and games for smartphones.
Their Diablo Immortal page currently shows 99,114 pre-registered players.
NetEase pre-registration page has 236,475 players.
I checked other Android app stores, and couldn't find Diablo Immortal in them. There were many other Android portals I found but they have no equivalent to a game webpage. You need to install the app to browse and search -- which I definitely won't do in my smartphone for obvious security concerns.
Just on those three portals, we can see there are a total of 1,379,037 pre-registered users awaiting to play Diablo Immortal.
We don't have the numbers of all the Asian Android markets, nor the western Google Play market; but we can assume at least 1/3+ of what we know.
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