One shark loses an average of 20,000 teeth in their lifetime. Some species lose up to 50,000 teeth in their lifetime. It only takes on average 7-10 days for the second row of teeth to move up to the first row. In 7 to 10 days later the first row of teeth end up on the bottom of the ocean where they land in a sediment and become fossilized. If you take 1 shark with 20,000 teeth times millions of sharks, times millions of years there is a number that your calculator would not be able to handle all of the zeros. The reason sharks have so many teeth is that don’t have hands to eat around the bone like we do. They bite right into the bone or even right through the bone. This causes damage to the teeth breaking them or dislodging them from the jaw. If they didn’t have replacements they would starve.