Thanks for great question! We should try to get all arguments down and insert resume into GrowFAQ.
When deciding number of plants per square meter there is no strict rule. It depends on growing style. You can go from Sea of Green in 4" rockwool cubes to single plant per square meter (middle bush). It depends if you have the time to vegetate them enough to fill the canopy.
I prefer square pots rather than round ones, because they can use the available space better.
There are also legal factors that limit plant numbers but most of the time it is just about the comfort of the grower.
Definitely pot size is different in soil where plant can get root-bound and in hydroponics where pot can be much smaller. Also see FAQ about choosing right pot size.