N = 1, but looking on Google Maps, pretty much all major parking lots around my suburb have at least 2 (small) trees per 20 parking spaces. In most large lots, the trees occupy their own islands, but a few lots have a line of trees around the perimeter instead. IIRC, there's a county rule requiring a certain number of trees on a property relative to the land area. An least in some properties, the issue has arisen of developers placing trees too close to buildings and other structures for their size.