anything upto 16 plants is classed as personal use depending on the equiptment you have!
if you have a basic tent n bulb system then youd be ok, if you had some crazy sog or auto feed system running they will probably class it as commercial grow! but 1-16 carrys the same sentence (usually a fine and possibly community service) ...anythin after 16 = jail time!
id suggest no more than 10 plants anyway as filling the room with plants is never a good idea for spacing!