You can put no more than 15,000 seeds in one pot. Thats the rule. If you dare put one more in, then you are in big trouble.
SOG is all about crowding. I do a form of SOG with 3 plants in a 3 gallon bucket. I FIM and LST the shit out of them so they dont get too high in veg. Then end up being this thick little SOG with 15 colas each plant.
The best way to answer your question is how big do you want your plants to get and how big is your planter. You never did give the size of the planter so your question cannot be answered with an exact number.
In my own EXPERIENCE. Do it. My outdoor grows are 5-7 plants per 3 gallon pot. Sure they are root bound. Yea they dont grow as big as they could, but bag seed, a bit of dirt and what not netted me 2.5 lbs last year. 6 pots, 25+ plants. So yea, it can be done and is done.