carpet pad is 85 cents sq. ft. how many pads do I need for 500 sq. ft. apt?

How many square feet is each pad?

Multiply it! That's beyond sad.

No way for anyone here to know. You would need to measure the rooms that you want to carpet. If you want every room carpeted you would need 500 sq. feet of carpet. If for example you have a kitchen that doesn't need carpet since it has tile for example, and the kitchen is 100 sq. feet you would only need 400 pads.

How long and how wide are the pads? Find out how many sq ft a pad covers (just ask the salesman) and buy accordingly but the carpet installer should know all this and probably can tell you how many you need (or just handle gettting them themselves).

Just one. Cut into the shape of your apartment.