Numbers not free
I Thought it was, apparently not.
iPad Air, iOS 9.3.1, Air 2
See the quote below from http://www.apple.com/mac/numbers/
Numbers is free on the Mac App Store for qualifying Mac computers purchased on or after October 1, 2013. OS X El Capitan required. Downloading apps requires an Apple ID.
If you are referring to iOS http://www.apple.com/ios/numbers/
Numbers is free on the App Store for qualifying iOS 9 compatible devices with initial activation on or after September 1, 2013. See www.apple.com/ios/whats-new/ for iOS 9 compatible devices. Downloading apps requires an Apple ID.