Everyone will have a different judgement for this because you can use different pot sizes. The main key here is filling up your room by training (Either LST or scrogging in my opinion) could be done heavily on one plant vegged for a long time to fit that room, or you can do what most people do fill it up a space like that using 4-8+ plants since the veg time is a lot quicker.
You'll get the most weed per watt if you're whole room or the area your light is focusing on is filled to the edge. I do think its nice to have some space in a grow room for fans and it allows air circulation, temps, and humidity easier to manage,
Example: If you have enough light for a 4x4 room, might be nice to have a 4x8 just to have room to have strong fans, electrical stuff, and other gear on the side that isnt being used it give the weed more fresh air to use since the room wont be full of plants. If you have CO2 great intake fan letting in lots of fresh air this shouldn't be a problem for a room tight to the brim with plants.