I think I would do 2-3 seeds per cell and then plant from that tray right to the garden come time with no interim transplant. Not all the seeds will germinate and once they settle in the garden you can harvest some of the closer ones for green onions to improve the spacing and let the rest go to bulbs. I assume you are using long-day varieties since you are starting them this late?

Dave

The best way to get onions is probably to buy plants. Less than $10 for 3 bunches of 60 to 75. That said I started mine about 2/4 hoping they will be big enough. I just spread them in cut off milk jugs and usually they are fine until transplant. Keep cutting the tops to about 3-4", which will make the stems thicker. I use 1 pkg per milk jug and usually have more than I need at transplant time.
JMO,
Tom

I have bought onion plants in the past, but want to try seeds. last year I did so unsuccessfully, and resorted to plants when the seedlings didn't work out. I hope to do better this year!