When the seedlings had produced their first couple of true leaves (not those kidney-shaped cotyledons, or "seed-leaves") I transferred the best ones to individual 3" or 5" pots.
I now have six each of two different varieties of Purple Sprouting Broccoli ("Red Arrow" and "Rudolph") along with six of Cavolo Nero "Black Tuscany". This is more than I will be able to grow. I expect to use only half of these, but I always like to have some extras to insure against casualties.
I decided this year not to grow any white Sprouting Broccoli, but to try some Flower sprouts "Petit Posy" instead. Unfortunately I sowed them too early and they are already pretty big. I now have the challenge of keeping them in good condition in their pots until such time as I can find somewhere to plant them out. It is supposed to be the place where the Broad Beans are now, but the Broad Beans are later than usual and it will be some weeks yet before they are ready to come out. I'll probably end up transferring them into the big pots currently holding my potato crop.