Simon Burney's 2 books , 1990, & 2nd edition 1996 go into training, techniques and Tactical stuff.

in 1st edition he went out on a limb and suggested 6 speeds mucked up to the point of stopping, Less.

Less than 7, or 8 , and as we see the market insisted on selling us 9, 10 and now 11,

but, the Mud hasn't changed.