Hi Ruth,

It really matters how quickly you want a dense planting vs. how much space you have. I'd probably wait until they had 6-8 good leaves before planting in the ground, though. You can space them as close at 6-12 inches for smaller seedlings to as far as you want. Ideally, mature plants look decent anywhere from densely planted to about 2-3 feet apart.

Craig
From Sacto (Ex So Cal)