The time to do this job is when the plants have formed their second true leaf (you discount the cotyledons or seed-leaves).
I am intending to grow only four Brussels Sprout plants this year. Last year I felt that six in one raised bed was too many. The plants did not really have enough room to develop their full potential, and they produced small sprouts.
Allowing for casualties, and to give me a good choice of plants, I have kept 3 plants each of 4 different types, giving a total of 12. The types are "Brilliant", "Cromwell", "Napoleon" and "Bosworth".
However, in my usual fashion, I have not yet discarded the others! I originally sowed about 10 or 12 seeds of each type, so I have kept 3 more of each in the original pots, just in case the pricked-out ones don't "take". I will discard them in 10 days or so if everything is well.
In similar fashion I have pricked-out 17 Lettuces. These ones are "Devin" and "Cervanek".
During the daytime these little plants will be kept outside in the fresh air, but at night time I will move them into the mini-greenhouses, because the nights are still very cold. Notice also that I have put a few slug pellets in each pot, because tiny lettuces like these are very attractive to slugs.
At this time of year I do a LOT of moving plants to and fro!