It is going to depend on exactly what you are growing and how large they get. I have seen some autos that barely get a foot tall and have little to no side branching. I have also seen them that get 3-4' tall and bushy.
Overcrowded plants WILL yield less than plants that have enough room so that the entire plant gets light and has air movement around it. I would start fewer =plants rather than more and learn ;how to grow. There is a learning curve to this growing thing and more plants does not translate to more yield. Yield is more a function of your lighting--I.e. you are going to produce x number of grams with the lighting you use, regardless of the number of plants you have (within reason of course). And your yield will increase as you gain more knowledge and get your space dialed in.
As far as LEDs, cheap LEDs are not good and good LEDs are not cheap. How much are you planning on spending on lighting? A good LED will run you hundreds and you are going to need 2 as you have a rectangular space.