that is very strain specific I would say I have a bubba kush going right now and i feel i could easily fit 10-12 in there on the other hand i have two heavy duty fruity that are just huge so I def think it is strain specific
How long do you plan on vegging? If you plan on taking a plant to maturity (5-7) weeks then you can get over crowded in a hurry. Depending on your growing style you may only need 2-4 plants. If you grow a quick maturing, vertical growing plant then you can fit up to 10 or twelve. The real question is how would you prefer to grow.
There are simply too many variables to really give you a concrete answer. Strain, training methods, and length of veg time can make the answer be a little as 2 or as large as 16 or more if sogging.
I have a 20 sq ft space and feel that 8 is the max number that fit in there without becoming overcrowded....and even then, they sometimes do. But I like to let my plants get some size to them before I flip. It is a lot easier for this old lady to take care of 8 plants than 20 or 30. You will be able to fit more indicas than sativas. However, for a general answer without knowing a lot more about the variables, I'm pretty much thinking the same as pcduck--5-6 plants would probably be great.
I would highly suggest some Larry OG Kush. My hands down favorite. Take it at barely showing 5% amber for a nice even head, very right brain, artistic, get up and do something high. Go about 10-15% amber for balanced body stone that is relaxing but still lets you get up and do stuff. More amber for more pain relief.
I would run no more than 4 plants in a 4x4 and do it under scrog for maximum yield. In about 6 weeks I will be putting mine under screen I can not wait