For some specific Marks I've been trying to keep a detailed record of the extant cars. Current number of cars that I know or believe to still be around:
Humber Pullman Mark II: 8
Humber Pullman Mark III: 16
Humber Pullman Mark IV: 5
Humber Super Snipe Mark II: 21
Humber Super Snipe Mark III: 28
If you have a Humber I'd like to hear about it!