For what it's worth (which isn't too much) Dropships & Jumpships, which gave us the number of 2000 jumpships in the IS, gives us an estimate of 25,000 dropships- or roughly 12.5 dropships for every jumpship. This number doesn't seem too unreasonable as there would a lot of dropships that don't or rarely leave their home system (either intrasystem transports or military dropships used for system defense.)