feistel cipher and number of rounds In feistel cipher , does it matter that number of rounds be 16 ? is using 16 rounds will give the best result ? Or using any number of rounds is the same? 

in cryptography and network security by william stallings we have : "The  essence  of  the  Feistel  cipher  is  that  a  single  round
offers  inadequate  security  but  that  multiple  rounds  offer  increasing  security.
A typical size is 16 rounds."

a typical is 16 but what's the efficient number ?
There is a minimal number of rounds for theoretical security. 4 rounds are required to construct a strong pseudo random permutation (PRP) from a pseudo-random function (PRF).

In practice, cryptographers don't tend to design ciphers with a heavy PRF for the f-function but instead apply more rounds with a weaker f-function to get the requisite strength. 

For this reason, there is no typical or default choice for the number of rounds in a Fiestel cipher. The optimal choice depends entirely on the internal security properties of the key schedule and F-function for the specific cipher in question.