How many sanding sheets do i need for a floor sander?

If you are renting the machine, the supplier should be ok with giving you a generous estimate of the paper you are apt to need. They sell more this way. Then return the rest. The disk sander is a bit tricky and you should probably try it out on a piece of scrap to help get the feel befor starting the floor. It helps also if you have fairly strong arms and hands. It is easier to control this way. Not sure about the bags. ask the rental person.