a lap is one length of the pool regardless if it is 25 yards, meters or 50 meters. You wont find many 50 meter indoor pools but there are few outdoor pools that are 50 meter you just may have never noticed it. In the swimming world people usually dont talk how many laps they did they just use distance. Most high school age swimmers do 6-10 thousand a day with a few exception either way. The elite swimmers can do upwards of 15K+ a day. If you are doing 500 a day I would increase your swim time and pick up an extra 200 a week.