At 300 replacements a year, the 18,000 pieces in the Tower are replaced completely every 60 years. Today, the Tower is 118 years old, so the Tower has now been "replaced" almost twice.
One more thing: the Tower was a gift to the City of Paris and the bequest specified that Paris could not sell or dispose of "any part" of the tower without permission of the corporation that paid for its original construction. If they were to do that, ownership of the Tower would revert back to the corporation. As a result, the old pieces removed from the Tower – the number is around 35,400 now – are all in a warehouse outside of Paris and their number is growing by 300 per year. Presumably, you could reassemble these "used" tower pieces into another full-size (albeit rusty) Eiffel Tower and still have plenty left over.