If you would like to look at the official java conventions: http://www.oracle.com/technetwork/java/codeconventions-150003.pdf
at section 5 it says:
"Note: The frequency of comments sometimes reflects poor quality of code. When you feel compelled to add a comment, consider rewriting the code to make it clearer."
What part don't you understand. If you really need something to explain the process when re-looking at it then add a comment. That's why I stated "as many as you want" because if you really can't understand what a single line of code is doing then you need a comment.
