The Cintiq 21UX2 has 16 buttons, and 2 touch rings (each touch ring is a scroll-wheel plus a button, so 3 buttons each), totalling 22 "mouse buttons". The code in gnome-settings-daemon only seems to support 16, as does the X.org driver. Needs testing.
Buttons > 16 are sent properly from the Cintiq 21UX2. XEV shows the final button emitting button 22 as it should. That said, buttons > 16 cannot be re-mapped to other buttons. "xsetwacom set <id> button 17 key a" continues to send button 17, while trying buttons 18 and higher produce an "Invalid offset into Wacom Button Actions property." error (since the property only contains 16 items, regardless of how many buttons are present).
http://sourceforge.net/mailarchive/message.php?msg_id=28780286
commit 7e92aca68a666499c7d6f1946cc849cfb5653ae0 Author: Peter Hutterer <peter.hutterer@who-t.net> Date: Fri Feb 3 19:15:54 2012 +1000 Initialize button action properties for the correct button number (#45557)
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