Hi Thomo,
The Humber 4 x 4 Heavy Utility was the only 4 wheel drive utility built by a British Manufaturer. It was affectionately known as the Humber Box. With independent front suspension the Humber chassis was a comparatively sophisticated design for a military vehicle. They proved to be very sturdy and reliable vehicles and many remained in service after the war.
The same chassis was also used as a basis for the 8cwt personnel/GS truck and light ambulance and also the Mk.IIIA light reconnance car.
I don't have any specific production numbers but it was used in all theateres of war:
Here's a link to the vehicle spec
http://books.google.co.uk/books?id=MuGsf0psjvcC&pg=PA107&lpg=PA107&dq=Humber+Box+4x+4+Utility&source=web&ots=K4UBBkUI7D&sig=P5ZZAGEg_53lV0YMDxaBuGiTtrM&hl=en&sa=X&oi=book_result&resnum=4&ct=result#PPA107,M1 Production began in May 41 and it was employed on a wide scale I believe.
Here's another link
http://warandgame.wordpress.com/2007/12/24/british-military-transport-wwii/ So in answer to your question, don't know the specific number but it was produced in large numbers and widely used at all levels and severed in all theaters of war.
Sorry I can't be of more help.
Al