tl;dr: Get the specs matrix for iPads here and a printer-friendly version here. You can find the version for iPhones here.
I still remember the early days of iOS (iPhoneOS!) when developers used to buy every new model to test their apps. I don’t know anybody who is still doing that today, where Apple is releasing up to 9 iPhones and iPads each year. It wouldn’t only be crazy expensive, the differences between some models are totally negligable from a developers point of view.
But you still need a few test devices. And that’s where I see the main purpose of the specs matrix, helping you pick test devices that support all features that your app requires, but still offer a variety of screen sizes, RAM, CPU and graphics performance, biometric sensors and other hardware features that you can’t really test in a simulator.
Please post any error you find in the comments! And yes, the benchmarks for the brand new iPad Pro’s are missing, because I can’t find any at the time of writing. I’ll add them to the next version.