Why would you only send 500 bombers when you have far more available? I suspect you're not getting the point. Send all of the bombers all the time, or what's available to you through serviceability and losses etc, and continue to do so. Send all on the first day, then continue overnight and send the same the next day. The secret to strategic bombing of any target (not just cities) is maximum destruction, and you can do that whichever way you choose, but just stipulating one day's operation is limiting your potential.