Groups of cyclists should be limited to ten per group and should leave a gap of ten metres between themselves and any group ahead.
Two abreast on minor roads if it leaves four(?) metres free for overtaking
or, when the road is wide enough for a centre marking line, any number abreast to the left of the line.
In my experience it is very rarely safe to overtake even a single cyclist if a car is passing in the opposite direction at the same time.