XmTabListTabCount(3) on Linux
The text 9fd479b191a7, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A convenience function that counts the number of tabs "XmTabListTabCount"