4.
I sometimes pot up from the cell trays to 3" square plastic pots, mainly when it's still too cold or wet to plant directly into the ground, or to give the seedlings more of a head start against the Slug Army. I would do more of that if I could find a place to keep MANY trays of pots - like more area indoors with light, or some kind of slug-proof outdoor cold frame.
I used to pot up from cells to trays of plastic Dixie cups or Solo cups, but they really tend to tip over (and I don't have room for them anywhere).
Inserts and propagation plugs don't seem quite deep enough, but big Solo cups seem taller than ideal. But I could get used to that if the @#$%&# things didn't tip over so much.
(P.S. you can make waterproof indoor "trays" from cardboard boxes by lining them with plastic bags. The boxes & bags can be reused almost forever, but never put a cardboard tray onto the ground in a rainy climate. They turn to mush overnight.)
