More plants do not mean more food. They mean smaller food, and then no food.
Red Beet is the clearest case: each “seed” is a cluster that throws up several seedlings. Leave them and you get a knot of flat, deformed roots. Thin to one and you get round beetroot.
Carrot Chantenay needs thinning to about 8 cm, and it needs doing on a damp, still evening. Bruised carrot foliage smells across a whole garden, and carrot fly finds it. Thin, water, and take the thinnings away with you.
Broccoli Calabrese and Cabbage Purple at 45 cm apart look absurdly generous in April and exactly right in August. Crowded brassicas make small heads and bolt early — the single most common way this crop disappoints.
The exception is Lettuce Mix, which is sown thickly on purpose because you are cutting leaves, not growing plants.
