In the UK under the so-called "Britain" branch there were about 8 Districts, I think - certainly no more than 10 if you include Ireland (Eire) the middle east and the outlying islands of the old empire.
In 2014 when the DO's were given the boot it was also the no-one over age 70 rule brought in so this is what happened.
About 6 or 7 DO's were over 70 and so used/sidelined as Drearyweather said. A couple of younger ones - (there was a "brother" Black - self important piece of work - who became a run of the mill CO - might be approaching 70 soon so good riddance there) also (Steven Clarke - one the few nice guys with a nice wife who was "only" in his 50's. I think he is a CO now.)
The DO used to shadow the CO twice a year (remember the DO/CO joint visits?) and now this is done by either a Branch Rep - we had a branch committee member shadow an "experienced CO" in 2015. Or an "experienced CO" will shadow a less experienced CO.
A note on "sub" CO's:-
They aren't subject to the 70 year old rule. So a booted "over 70" CO can sometimes be used in this role if he has been booted to the curb as a "special pioneer" and they don't want his sorry ass in Bethel and have dropped him in on some unsuspecting congregation as a CoBE or somesuch!