It is certainly in the thousands, if not tens of thousands - depending on the size of the roof. Not to mention that when the old roof is removed, all those nails are removed too! None are reused. Roofers have to work hard to ensure when disposing of the old roof that all the nails that come off are properly cleaned up from the yard and disposed of. Its a tedious but necessary step and magnets are a roofers best friend. It certainly helps to have the lawn cut before removing an old roof, so the magnets can better reach any nails that may fall in the yard.