I'm currently at 6 for my own use. Planning on replacing the 12.9 iPad when the new one comes out. Might keep the old one for beta testing.
iPad mini 4 128GB Wi-Fi - iOS 10, used for 32-bit apps
iPad Pro 9.7 256GB LTE - iPadOS 13, used for abandoned apps that have gotten even buggier on 14, wish I'd kept it on iOS 12
iPad Pro 12.9 (2017) 512GB LTE - docked third screen at the office, home-use is mainly reading American comics
iPad Pro 10.5 512 GB LTE - 2nd most used iPad, used alternately with the Air 3
iPad Air 3 256GB LTE - most used iPad mainly because it has the best battery life, EDC/purse, used alternately with the Pro 10.5
iPad 7th gen 32GB Wi-Fi - cheapie model ($229) left in the office, used for side-by-side PDF annotations via Logitech Crayon along with the Air 3