I hate when people answer a question with a question, but when you say onions, you are doing plants or sets, correct....not seeds?
To be quite honest. I am not really sure. In my burpee seed starter kit, I have about 3-5 seeds in each little square and I sowed several squares. I am assuming that I will get that many plants.
When time to plant I was hoping to have learned more information by then. LOL
I guess I really don't know how what is meant by "plant" on the EB chart. Does 1 seed = 1 plant?
Onions take a very long time when started from seed (they need to be sown a good 3-4 months before spring.) For that reason, people usually buy starter plants or sets. Sets are small onion bulbs that actually are 2nd year onions. They are also the easiest to grow.
Onions have a built-in alarm clock, based on day length and soil temperature. When their alarm goes off, the onions are done... makes no difference when you planted them... they will not get any bigger than they are at that point. The foliage falls over and turns yellow. In order to get decent sized onions, they need to be planted as early as possible. Sets must go in by early April at the latest.