I have planted my 3 Brussels Sprouts down the middle of the bed, spaced about 50cm apart. They are one each of "Napoleon", "Bosworth" and "Brilliant". Even when you're growing a small number of plants it makes sense to "hedge your bets" and have plants of several different varieties, so that if one type doesn't do well it's possible that another will. My three will mature at different times as well, which spreads the harvest over a longer period.
As you can see, my seedlings had reached the stage where they had two pairs of proper leaves, which I think is the ideal size for transplanting. If you leave them in little tiny pots for too long they can sometimes run out of nutrients and go weak and spindly.
Each plant is protected with a home-made collar cut from scrap cardboard, offering some protection against Cabbage Root Fly.
Once the planting was complete, the net went back on, to keep the butterflies off. With the advent of warmer weather these last couple of days I have begun to see the odd one or two butterflies, though thankfully no Cabbage Whites just yet. The ones I have seen are mostly the yellow Brimstones.