What Percentage of Android Devices Run on a 64-bit Architecture?
I have not yet been able to view recent statistics, but
according to this article published end of 2017:


  The ARMv8 instruction set architecture rolled out 64-bit hardware support in 2014, and now almost every new Android device can run 64-bit code (but only 40 percent of active devices)


From this ARM's blog post:


  Nearly 90 percent of today’s Android devices deploy a 64-bit capable version of the OS (version 5.0 and above).


Statistics on the percentage of OS used can be checked here and here.