I started celery from seed once, in a 4-inch pot, and planted way too many of the dust-size seeds. Celery is slow, but I potted up in batches when it was iirc a couple inches high -- by then it was getting pretty crowded. I had so many plants that I ended up giving most of them away.
Since then I've let celery go to seed in my gardens, and I always have stray seedlings in my plots and in containers.
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