I've lights on at side of my loft from flats , lighting whole garden up birds still rest no bother at all , guess how many you need will depend on size of each section and where your getting the most sun from a Sheet of plastic towards bottom of roof will cost less to do a large square leaving first row or two at top of roof depending on height , you can always add a sliding false roof , so can slide so can block street light out and reduce sun in to loft as well , have seen a few lofts with this , I think one of Chris Vasey's lofts is like this .