I have heard that iOS 11 will support only 64 bit apps.
But I don't quite understand because my iPhone 5 does not have a 64 bit cpu so how will it work?
If an iPhone5 can't run 64 bit apps, and Apple say they will refuse new 32 bit app in the app store starting in January 2018, how can I continue to maintain my app for my iPhone 5 users after 2018 ?
The iPhone 5S has an A7 chip. This is a 64 bit CPU, which is why it can run iOS 11. An iPhone 5, however, cannot be upgraded past iOS 10.
You will still be able to submit apps containing both 32 bit and 64 bit slices and, so as long as the minimum iOS requirement of your app is < iOS 11 the app will still run on an iPhone 5
You can no longer submit 32 bit only binaries. Older, 32 bit only, apps give a warning in iOS 10 and will not run in iOS 11.
After January 2018 you cannot submit a 32 bit app to the mac app store