The answer depends entirely on which square mile you're talking about, how long and hard you look, and how good you are at finding things! A square mile of forest or estuary, for example, will have many more species than a square mile of cornfield or parking lot. A square mile of mixed habitats will have more biodiversity than a square mile of uniform habitat. You'll find many more species in Massachusetts in summer than you will in winter. An experienced observer will find more than a novice. It's virtually impossible to do an exhaustive inventory of the fauna of any square mile; something inevitably will be overlooked. The more time you spend looking, and the more methods you use, the more you'll find - but you'll NEVER find it all!