The number of teeth a shark has depends on the species of shark. For example, the great white shark has about 50 teeth in its mouth. But the amazing thing about sharks is that they can replace damaged and lost teeth very easily. Below the teeth they have in their mouths, there are several rows of new teeth waiting to replace the old ones. It can take anywhere between a day to a few months to replace the old tooth. So, amazingly, a shark can go through about 30,000 to 50,000 teeth over the span of its life. Here are some helpful websites with more information:

That kind of number is going to change from shark to shark. The point is that sharks keep growing teeth throughout their lives - no matter how many they lose, they will never run out.

Typically a shark has two to three working rows of teeth with 20 to 30 teeth in each row, although a whale shark has about 300 teeth in each row. The replacement rate has not been measured in most sharks but normally the teeth seem to be replaced every two weeks. The lemon shark replaces its teeth every 810 days, and the great white shark replaces its teeth about every 100 days for young sharks and about every 230 days for old sharks. (From wikipedia)