How many plants is too many?

Its not too much... Look up SOG (sea of green) grows. They will help you to accomplish what you're looking for with multiple plants in smaller spaces. Although, unless you're growing multiple strains I myself would probably stick to no more than 4 plants in a 4x4 tent to give for plenty of spacing and air flow... essentially give each plant its own 2x2 space in your tent but I've always been a less is more kind of guy. Either way happy growing! ;)

I have 15 clones in my 1.25x1.25 you can have as many as you want its just controlling that many that can become an issue. Scrogging is usually done with just a couple of plants but theres no reason you cant have 6 I would advise 4 though if they are seeds/ If your doing clones then you can have as many as you can fit pots in have a look at my sog for pointers I have 15 dwc buckets all with clones it and a nice canopy :)

I grow 2-3 plants in scrog. You can grow up to 4, and really you can grow as many as you want, but you will have alot of growth with that many. I grew 5 once and was too many for me in that small of a tent.