ok heres my suggestion you could probably put 10 if you did the SoG method, just remember to get small netted cups all they have to do is be able to hold up a plant which really doesnt way much at all. If you just grow them normally I would advise that you cut the bottom leaves off as it gets thicker and thicker in the canopy, this will help with airflow which will make the plants thrive. what you have to remember is that if they are to close the bottom leaves wont get much light no matter how you hang your lighting, with SoG technique it gives the plants a brace to space out on. Spreading them out just allows the light to reach all the leaves, i have an 18 gallon resevoir and unfortunately i bought the biggest net pots possible and i could only fit 4.