planting sorrel from seed
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I'm assuming you are growing the perennial herb sorrel. The short answer is yes you need to thin them to a single plant. Spacing is generally about 12 inches in the garden.
To thin out your seedlings you have a couple options. You can try to separate them before they get too crowded and pot them in separate pots or you can just cut off the seedlings leaving about three per container. Leaving three gives you a bit of an insurance policy when you set them out. If all three from the pot start to grow well outside you will need to cut off two leaving one. You can use the cut sprouts in salad.
Here are a couple links you might find helpful. https://extension.umn.edu/vegetables/growing-sorrel