Answer: A five inch pot, even with just one lettuce plant, will dry out pretty quickly, so you'll need to keep a close eye on it and water it regularly. If you can maintain even moisture, you should be able to harvest some leaves. A larger pot would be better, though. I wouldn't keep more than one lettuce plant in a 5" pot. You can try transplanting the lettuce seedlings--but you'll have to do so very gently. It would be easier to get another pack of seeds and start over, and simply snip off all but one of the lettuce seedlings in the current pots. But I know that's hard to do!
Remember that lettuce generally prefers cool temperatures, and will bolt to seed in hot, dry weather. Try to keep the plants in a cool spot, out of the mid-day heat and strong sunlight.
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