I planted 15 or so varieties of tomatoes in one large planter. Cut the bottoms out of yogurt cups & used them to keep the varieties separated.
& as Ronnie said, think outside the box...or inside, even. One year I got desperate & used a cardboard box for a container. Worked fine, but I learned not to try moving them. But if it will hold a couple inches of dirt, it's fair game for me.
