Donna, I am not sure what you mean for "seed plugs". I use plug trays and seed them 1 seed per plug (or square). I get a better idea of how many seeds germinated and a quick count on how many I started if I forget to write it down.
I agree with Carolyn, you can use plug trays to plant seeds. I do! Just don't pull the plants out as soon as they germinate because if you pull it out, then the dirt won't come with the plant, you will need the dirt to transplant your seed. Make sure the plant has roots before you go ahead and transplant them.