How do I tell which tomato plant seedlings are the best and which ones to pull out?

They absolutely have to be separated if you have them all in the same cup. By the time you get them apart and in individual pots with drain holes-not more cups, you may have twenty seven that will survive. So, no problem. To keep them growing simply put the individual pots under your grow light for twelve or fourteen hours a day and keep the soil in the pots slightly moist.