Individual seeds are cheap, and excess seedlings can easily be composted or given away.
Number of Seeds: Aim for 1-2 plants per cell. You can either cut the weaker plant off at the soil line after they germinate, or separate them when it comes time to pot them up or transplant them out. I always sow more seeds than I need. Germination is not always 100%, and it feels like a waste of time to wait for seeds to germinate, only to have to sow more if there aren’t enough. Individual seeds are cheap, and excess seedlings can easily be composted or given away.