Nice! My favorite exercise! If you can do 5-6 I would recommend that you try to do 10 sets of 3 or 4 pull ups with 20 push ups and 20 squats in between as rest for your pull ups muscles. I do 10 sets of 10 pull ups, but I can do 20 pull ups in one set... that is why I reduce the multi-sets to half that number. With time doing this you will get much stronger and your single set max number will go up... I am just over 100 kilos so it is not like I am a light person that can do many pull ups...