This reply will address overall space you have to deal with. The best way to demonstrate what a tent will fit is visual. My tent is a Covert 3x4 with 6’ height. The side ‘veg’ chamber takes up a foot of the four feet so really growing area is 3x3 as there is a bar at the floor that makes it hard to put a pot on. My tent at present has five autoflowers in it, all came up around first part of September so I am halfway through the taller ones in back and the shorter ones will go about six more weeks. I have four in AutoPots and one in a SoftPot, over in the veg chamber area. I tend to overgerminate and then can’t bring myself to kill a plant so stick it to the side. I battled tent space last grow. Today I am thinking next grow may only be two plants.
Space with height to consider, and this is rarely covered: calculate the height from the top of your container to your light at it’s highest point you can raise it. Calculate the prospective height of your plant. Calculate the distance from the light to no less than 12" from it, that will be the absolute MOST you will be able to allow your plants to get to prevent light burn. I like mine to stay around 18". In my tent that gives me 31" of total plant height to deal with tying a plant down and some plants do not like manipulated. So you have to do some research and cross reference. Seed banks sometimes do not put all the description about a plant on there. Nothing irritates me more than seeing “average” or “tall” for a plant. I NEED a bit more info such as 90 cm - 120 cm in prospective height. If that makes sense. So this will be your guide on choosing strains. I applaud anyone who can grow a ‘tall’ sativa in a tent. I would suspect in my tent that would be ONE plant and I would end up wrapping it all around the tent.
So to answer your question on adding plants: you can. But you will need to get creative not to mention brave, on how to manipulate plants.
So for reference. Today’s photo of the grow shows a packed tent.
Photo of first grow shows the elaborate lengths I went to to make one plant fit. Originally had seven in there and ended up with five. Four smaller ones and the big one.
I hope this helps you and makes sense!