How many do you need or how many would you like to have? I have 4 on 160 acres right now and am thinking about getting another. It is intersting that I have some pictures of a particular deer on one camera but not on any of the others. You could probably get a good idea of your population if you just moved one camera around a bunch though. I like to use my cameras to tryto get a feel for behaviour patterns but it is really pretty hit or miss most of the time.