Tim - how much longer will the plants be in the cells before you are ready to plant outside? If it won't be for a couple of weeks, I'd say you'll have to thin them to 2 or 3 per cell or you may end up stunting the whole batch. Lettuce seedlings do transplant easily, so you could try to tease some more of them out into other containers if you want to save more. But if yours are very crowded with the roots enter-twining, you may have trouble separating without snapping a few. I try to get them outside into natural sunshine as soon as I can, even if I have to bring them in at night. My lettuce under fluorescent lights tends to get pale and lanky and finicky to harden off.
I normally don't put more than 2 or 3 seeds per cell if I'm starting in the house, then separate and transplant the extras to other cells, leaving one plant per cell. Seeding outside in the garden, I sow pretty thickly, knowing that I will thin and transplant the extras after they get about an inch high. It's surprising how much root disturbance they can take when grown outside. They may look wilted, but will perk up in a few days as long as you keep them well watered.
Dee
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