Seedlings of leafy crops need space, so thin them out now.
Before you start, check the optimum spacing for the crop you’re working with. A simple wooden batten with notches every centimetre helps.
You could choose to thin by half, harvesting every other seedling to use in a salad, and then finish the thinning in a few weeks’ time, or you could do it all in one go. It can be wise to do it in stages as this also gives you spares in case of mollusc damage.