That's an old wives tale.Yea, I agree. I wouldn't put more than 1 per pot honestly. Esp if you want to grow small plants and still yield 2oz on each. Those are going to strangle each other beneath the soil. One plant in that pot will prob give you more than all of those together.
There will not be a sub-soil no holds barred cage match "2 plants enter, 1 plant leave" type thing.
Will they reach their full potential in such close quarters ? Probably not, but they won't kill each other either.