I use two. They go across the entire body from post to post. They work great for me. So that is one in the front (post to post) and one in the back (post to post).
the pins skymaxx was maybe joking about would be cool. You'd just have to have the hole lined up accross the body. Two clips to do the work of four. But that might look ugly....
oh yeah I use 4, but using two is a good idea for tuning.