9 plants 3x3, but 16 plant 4x4 with 6 to 7 l pots would be even better imho in SOG
https://www.royalqueenseeds.com/blog-how-many-plants-to-grow-per-square-meter-n774
Hey buddy, I have 6 plants in a 1x1 space. You can check out my grow and let me know what you think :) I think it gets a bit tight at certain times, but it's manageable to grow without any issues.
Hugs and happy growing!