there is no set number on how many per acre
most guys work with there budget, and buy cam's as extra allows
I at one time had about 25 cam's spread between a few properties I hunted
I had a LOT of issue' with things being stolen, from, stands to camera's to property stored on site in farmers buildings
was all stolen by trespassers
but was almost impossible to prove and have them arrested
as for what's the best cam for the $$
I think all the cheaper cam's are hit and miss, some guys buy and love THERE"S and it works flawless, and another guy can buy exact same make model and have nothing but bad luck with them
Higher end models seem to be more consistent with working good or NOT
I got my first cam back in the 90's a 35 mm film job and been playing with them ever since
IMO< the VERY best OEM cam and metal box, is made by cuddyback, its the ONLY metal box on the market from cam makers ( That I'm aware of) that makes the metal box so it also protects the lock and NOT just the cam
all the other boxes, any idiot with bolt cutters will just cut the lock off and bye bye cam and box
they also have super fast trigger and wake up speeds
I have some cuddyback cam's that run 24-/-365 for 8-9 yrs and still ticking(also have a few that died LOL)
I have had most brand of camera's over the yrs
and honestly would say ANY model is best, minus the very HIGH end price tag models , some of them have been flawless, but there far from cheap!
and due to so many trespassers and things growing legs I cannot justify buying a lot of them
MY cuddybacks, in a metal box have NEVER been stolen, BUT I have had a few boxed beat up with a baseball bat/large stick, and that killed the cam in the process
so NOTHING is full proof
and YES< I have also hung cam's 15-20 ft up., and had folks come back and bring a ladder or cut the tree down
NOT living on site has many disadvantages!!
god luck!