well first whats the shed made of?
how do you plan to cool that thing if the sun is beating down on it and the lights on too. as for number of plants.. 8 seems fine, theres long time members here that COULD fit nearly 100 plants in there, that really depends on strain and grow style. Do not buy mylar, shit sucks, get some panda (black and white poly). lights, well that depends on how you plan to cool the shed. as for being out in the open, well.. how you going to get stuff in the shed, water your plants and alter the sheds out side to accommodate for ventilation without arousing one persons curiosity?
final point i would like to make is this, if im curious about whats going on in your shed and i think its illegal, it only takes 10 minutes and some bolt cutters to get in your shed. Ya fell me?