The best answer is- well- depends on what you are looking for. ie I grow medical and want my colas close to the same size and potency, so I am unlikely to trim cola's that are getting equal light.
That said, when I have kept only 8-10 cola's I got some amazing results with larger buds. I can not say my yields were more- just more impressive to the eye.
Here is my timeline of when to trim:
lollipop the plant going from Veg to flower.
Then trim on weeks 1 & 3 in flower, if needed.