Question

That's a lot of rhubarb. According to the KGI Garden Planner, you need roughly 3' x 3' per plant, but in practice you can grow rhubarb more tightly spaced than that if you don't mind them hanging over the edges of your raised bed box. I would try two plants and plant a 4 ft row of something else on the south side of them so the rhubarb doesn't crowd and shade them out. Maybe a few strawberry plants if you want to make yourself a delicious pie!

Good advice! Those are good-looking rhubarb plants!