How many plants in a 70x70 tent

I would do 4 smaller ones as you're limited with height. Or one large plant if you are experienced enough to train it .

the area dictates yield - in addition to how you fill it, of course, but it is the cap on potential. more plants in the same sized area will speed up the process but not provide a greater yield, all other factors remaining the same. if you more efficiently/effectively fill the space, you'll potentially improve your yield, but at some point you can only produce so many grams per sq ft no matter how good you are at it. more plants will reduce required vege time. if they are autoflowers all you will do is overcrowd the area or have to prune off a lot of viable growth... which equates to wasted vege time until it decides to go into bloom.

It really does depend. 1 plant (a monster if its weed you have a fondness for) no point growing big huge fems and have 100s of grams of weed you don't like. So, if you're strain hunting and doing fems 3-4 fems with a 4-5 week veg. 4 autos. May aswel put them in 4 big pots that fill your tent floor. As with autos you could have 100L pot and turn out you get like a 7inch girl with about 15g..

One plant only in that size.

4 or 9 plants in sog

For that size a tent I would stick with 2 plants at most, one of its rated to be a large strain. I grow in a 90cmx90cm and find I can max it out easy with two photo plants in 20L pots, or 3 auto (if med to smaller size)