One seed = one plant. It will start as one stalk. As it gets bigger that stalk will branch out. You can encourage that by once the plant has maybe 4 pair of true leaves (not counting the first seed leaves), pinching the top pair off. It will divide into two branches at that point. Once the two branches are growing well and have a few pair of leaves, you can pinch the tips of those off and then they will both divide, and so on.
Eventually, if all goes well, you have a plant that is still all growing from one stalk at the bottom, but is 3 or more feet tall and almost as wide and very bushy.
They don't really show you the base of the plant, but if you could look you would see all of that is coming out of one stalk that goes into the ground.