When you purchase starts from the nursery, there are often more than one plant in each cell. Before planting, seperate all individual plants by gently loosening the soil around the starts, and ripping the roots apart if need be. (A little man handling will NOT hurt the plants!) From there, plant individual plants with proper spacing. Lettuces/Bok Choy/Spinach need about 8″ between, Broccoli/Chard/Kale/Cabbage/Tomatoes – any of the bigger plants need 16-18″.
Here is a pic of how NOT to plant – the plants are too close and both are unable to come to full maturity.