An orbital floor sander manual should always specify how much sandpaper you need for a project.
Suppose you have a U-sand sander and want to work on a floor that’s 100 square feet. This project requires around 20 x 36/40-grit sandpaper (if your floors have Shellac), 8 x 80-grit discs, and 8 x 120-grit discs.
It may sound like a lot but remember there are multiple steps to a fine sanding job, and you have to change sandpaper grit a few times for optimal results.