If A8 gets iOS 13 (very likely given how recently Apple discontinued it,) A9 will most likely get iOS 14. A later processor version usually means at least one more major release than the one before it, with the obvious exception of the 3GS (using the last of the Samsung produced chips) getting iOS 6, where the iPad with the A4 stayed on 5 - and the 4 with its greater RAM but the A4 chip continued to iOS 7.
A5/A5X left on iOS 9 - Mini 1, iPad 2, 3 and iPhone 4S.
A6/A6X left on iOS 10 - iPhone 5, 5c, iPad 4.
Seems clear that iOS 12 will be it for anything with an A7. As yet the X chip hasn’t seen any further updates as compared to the standard variant of the chip, but I expect that to change with the Pros.
Things will get interesting when it comes to the A8/A8X, and perhaps A9/A9X (9.7 Pro having half as much RAM as the 12.9”, but a faster clock speed than the 5th Gen iPad also with 2GB RAM.) So far RAM seems to have dropped the iPad, while extra RAM (matching the iPad 4) but the same clock speed as the iPad 2 saw iPad 3 dropped at the same time as the 2.
Of course there are iOS releases which throw curveballs by not dropping anything (8 to 9 and 11 to 12) and we don’t know when or if there might be another. The Mini 4’s longevity, similar to the iPad 2’s might strengthen the case for another such update.