If the result provided by RNGCryptoServiceProvider were weak or flawed then hash of that result is no different than the original entropy and it too would be weak.
In other words by hashing it n times you aren't really making a meaningful difference. The correct way is to mix it with another source of entropy. For example some use Guid.NewGuid() and mix (eg. using a KDF) the returned 128 bit with the 256 bit entropy, you could use a user input, etc.