Artist Paul Kidby offered the following (incomplete) key to the 'Discworld Massif' on his instagram site. By my count, this is only 39 of the 77 characters depicted.
Featuring left to right, back row to front row, 'Nijel the
Destroyer','Moist von Lipwig','Adora Belle
Dearheart','Detritus','Samuel Vimes','C.M.O.T. Dibbler','Otto
Chriek','Cohen the Barbarian','Frederick Colon','Magrat
Garlick','Verence','Gimlet','Nobby Nobbs','Grabpot
Thundergust','Tiffany Aching','Mr Fusspot','Errol' and 'Of the
Twilight the Darkness'.
![enter image description here](https://i.sstatic.net/3LXtC.jpg)
Featuring left to right , back row to front row, 'Giamo
Casanunder','The Dean','Havelock Vetinari','Leonard of Quirm','Mort
Sto Helit','Death','Esmerelda Weatherwax','Ponder Stibbons','Mustrum
Ridcully','Rincewind','Gytha Ogg','Greebo','Twoflower','The Luggage'
and 'The Librarian'
![enter image description here](https://i.sstatic.net/aLmIe.png)
Featuring left to right, back row to front row, 'Ysabell Sto
Helit','Lord Downey','Constable Flint','Dr Whiteface','Susan Sto
Helit','Alberto Malich','Carrot Ironfoundersson','Angua von
Uberwald','Lupine','Lu-Tze','Reg Shoe','Eric Thursley','Cheery
Littlebottom','Igor','Snori Snorisson'and 'Gaspode'.
![enter image description here](https://i.sstatic.net/ogHqz.png)