According to Apple, as of January 19, 2015, 69% of devices — iPhones, iPads, and iPods touch — currently accessing the App Store are running iOS 8. That leaves 28% still running iOS 7, and 3% running iOS 6 or earlier.
The iPhone 4, originally released in 2010, is the only device capable of running iOS 7 that can't be updated to iOS 8. iPhone 6, iPhone 6 Plus, iPad Air 2, and iPad mini 3 all shipped with iOS 8.
Everything else, including the iPhone 5, iPhone 5c, iPhone 5s, iPod touch 5, iPad 2, iPad 3, iPad 4, iPad Air, iPad mini, and iPad mini 2, can be updated to iOS 8. It's reasonable to assume some portion of that 28% is comprised of iPhone 4. Tens of millions were sold between 2010 and 2014 and many are likely still in use. How many is impossible to say.
Presuming you're not using an iPhone 4 or older device, however, and your device is capable of running iOS 8, and you haven't updated yet, what are you still waiting for?
Do you not have enough storage space on your device to handle the software update process? Are you afraid iOS 8 will make your device run slower, or will be buggier than iOS 7? Are you extremely cautious and want to wait on iOS 8.2 before updating? Or is there another reason?
Let me know the reason in the poll above, and give me the details in the comments below. If you've already updated to iOS 8 but have relatives, friends, or colleagues who haven't, please send them the link to this poll so they can vote. I'd love to know!