Thanks to Apple's quality requirements, the total number of apps in the App Store shrank in 2017. In the beginning of 2017, there were 2.2 million apps. At the end, there were 100.000 less: 2.1 million apps.
Apple started telling developers that old, outdated apps which don't support new devices would be removed in September 2016. Apple has also created new App Store guidelines and all virus scanner apps were officially banned.
Changes in iOS 11 may have had the biggest impact on the available number of apps in the App Store, though. With iOS 11, Apple stopped supporting 32-bit apps, which the company began phasing out in 2013. 32-bit apps do not open on devices running iOS 11, nor can older 32-bit apps be found in the App Store.