One thousand chin-ups
I used to do a lot of chin-ups when I was younger. I had a bar at the doorway to my room and used to do a couple of chin-ups whenever I walked through it (or rather swung through it). At my best I could do couple of dozen chin-ups in one go. I think my record was 27. I was also able to do a few ones with 20 kg extra weight strapped to my waist. I was never really fit in general, it was just the combination of doing that regularly and weighting way too little.