The book says to cut back to two buds at least one third of the bush, plus any extra to reshape.   This normally works out to be about three or four branches all together.   Pruning is a bit of a scary thing as when you don’t really know what you are doing it feels a bit backwards to be cutting bits off in order to generate more.  Here we go then for this year, probably not really how it should be!   Last year, I didn’t cut the bush at all.