It's the only way I sow them, 1 seed in each cup.

I use a 12-16 ounce size. usually 12oz and 16 oz is harder for me to find.

I have found the plastic one end up melting with the sun on them all day, so use styrofoam ones.

Punch hole in bottom of the styrofoam cup with my finger.
Fill the cup about half full with good potting mix. put seed in center, then sprinkle with a bit of potting mix.
Then I slip a store brand ziploc baggie over the cup, pull it down, put slits in the top for air holes and rain to get in and out they go in a tray.

Once the have 2 "true leaves" on the stem of a tomato seedling, I slip the seedling out of the cup, add some more potting mix to the bottom of the cup, then put the seedling in the original soil back in the cup, add some more soil to the top to go up the stem.

This lets the stem of the tomato seedling develop additional roots before I plant them out.

Growing up on a summer veggie farm, my grandfather used to plant the tomato seedlings on their side. He'd cover the stem with soil so that the stem could grow extra roots and be a stronger plant. the leaf part would just aim for the sky and grow straight up and yes, he did have the healthiest, largest tomato plants around with the largest yield of tomatoes.

My method is just adapting his method to WS the seeds in cups and everyone around me says my tom plants are the healthiest and largest around.

Fran