This year I sowed my Brussels sprouts seeds on April 10th. They were one pot each of "Attwood F1" and "Green Marble F1". A few days ago I judged them ready for potting-up. I reckon this stage is reached when they have developed two proper leaves in addition to the cotyledons (seed-leaves), as shown here:
Brussels sprout "Green Marble F1"
You can see that this pot of "Attwood F1" seedlings is beginning to look pretty crowded now and the seedlings need to be given homes of their own.
Just to illustrate my point further, this pot of "Greyhound" cabbages, sown on the same day as the Brussels sprouts is not ready yet. The seedlings are still mostly very small. The two biggest ones would probably be OK to transplant, but not the others.
Brussels sprouts are big plants and take a long time to reach maturity, so in my small garden I can really only find room for about four of them (normally grown in the same bed as my PSB), so in my usual fashion I have potted-up eight, allowing for the possibility of some casualties as a result of Cabbage Root Fly or whatever.
To help me identify the two types, I have put the "Green Marble" ones in black pots and the "Attwood" ones in terracotta-coloured ones. This means I don't need to label them individually. When the "Greyhound" cabbages are ready I'll probably put them in square pots so that they are easily distinguishable from the Brussels sprouts.
These little seedlings are destined to be planted in the bed currently occupied by my Broad Beans, which won't be vacant for perhaps another month, so it's possible that they will outgrow these pots and need putting into bigger ones, but we'll cross that bridge when we get there, as the saying goes.
Broad Beans "Express"