How many plants per square foot?

That's it how many realistically in a 3x3 or 4x2,
Is there some formula or something?

Nope.its really up to you.you could cram 10 or 12 small 1 gallon pots in there with a short veg or put 1 larger 5 to 10 gallon plant in there and train it during veg to fill the spot then flip to flower.lots of tent grows in the led and cfl section to look at and see all the different methods.

i would do 2-3 small-med sized plants. but then again that is just me..

1 plant per 2 sq ft usually. That's in a 3 or 5 gallon.