How much space between plants?

How much space should I leave between my plants? I would like to utilize as much space as possible and get as many plants as I can, but I prefer potency over quanity. I know plant size differs greatly, but what would be a good rule of thumb?

4 plants in 20 ltr pots per m2.

it really depends on how long you veg the plants, the longer you veg the less plants you can do with out crowding. If you sog and flower when they are smaller then you could put more plants in smaller containers. I wouldnt go anything less than a gallon container

Why not SOG IT? Well if growing outdoors i would space two to three feet apart,believe me they will get bushy!Now if growing indoors a couple to a pot and grow them like small bonsai's that way you could concentrate on buds not vetical growth.