I grow my edamame in the ground and I space them 6 inches apart in all directions. I however always direct sow them in the ground. Any reason you are starting them in seed starting trays and transplanting them later?

They don't require any trellis as they grow more like bush beans. They may fall over when they are loaded with beans but it isn't a big deal either.

While I grow a lot of soybeans, I confess that I have never tried them in containers. However, a healthy plant takes up about as much space as a bush bean. The 6" spacing cited by Ikea will produce bushy plants. It's also woth noting that the healthier the plant, the greater number of pods with 2-3 beans... if spaced too closely, you will get a higher percentage of single-seeded "pollywogs".

Since soybeans have large root systems, my recommendation would be to allow about a gallon of soil for each plant. There are 231 cubic inches to a gallon. As a frame of reference, at 6" X 6" X 6" depth, the root area per plant at Ikea's spacing would be 216 cubic inches. If the depth of soil in the pot cited above is 5", you would have about 300 cubic inches... so for best results, I would recommend one or two plants only.