Two comments here:

• First, you assumed that we were talking about physical limitation here, as how many file hanlders the OS allows. That's one thing, but not the only thing we are talking about. I agree it is good to have this in mind.
• Second, with this physical limitation in your mind, you most likely don't want to go there, but rather stay somewhere below. I am not saying you suggested to go there, but rather a comment for the OP in general. He has to test and find out a reasonable number.