With exercises that you are new to, ones that you have never done before, or haven't done for several weeks, I would advise doing about 12 reps, and 2 sets. But that's with things which can be adjusted (weights). The amount of weight being used in an exercise determines the reps; you choose a weight which is heavy enough that you literally cannot do 13, only 12. That way it would be a fairly light weight.
However, with body-weight exercises like pull-ups, press-ups etc, you can't adjust the weight below your own bodyweight.
As you're already able to do 100 pullups in a day, that suggests you're in pretty good shape already, in at least one way (strong back, and low fat ratio, I could be wrong?).
With new body-weight exercises, I'd advise just doing what feels comfortable. Don't push yourself to your very limits straight away, you may injure yourself. Do 2 or 3 sets of good reps, stop the exercise immediately if you're shaking / struggling. Stop and wait a few seconds and try again, so you do smooth controlled movements. I've been doing this with one-legged squats, as I have had problems with my knees. I was wobbling all over the place, well, I still am (!), but I am getting better by concentrating on control. You will be able to do more reps as you practice the movements.
But really, with body-weight exercises, I don't think it's really possible to hurt yourself that much by doing too many reps. One day I did 50 squats in a row, and could barely walk afterwards, but I was fine the next day. Similarly, I did 300 pressups during the course a day, and really felt the burn the next day, but it was just normal exercise burn, not an injury. Your body should really clue you in to if things aren't going right.
Once you've done the exercises a few times and your body is used to the control of it, then you can step up the reps, or add weight, adjust the angle, or do them even slower to make it harder. It varies from person to person how long it would take to get used to them, but I am sure you'll feel it, feel what you are capable of doing.
I've been told to stand on one leg with my eyes closed for 1 minute to strengthen my leg muscles and learn balance to deal with my problems. That sounded so easy, but I sucked! I was okay with my eyes open, but eyes shut, I got about 14 seconds... but now after about a month or so, I can do 1 minute on each leg. It still doesn't come easy to me, but I think in a few more weeks, it will be really simple, you just have to keep trying the things that you struggle with.
I'm gonna start doing what you said, and then add some more. Standing on one leg sounds pretty easy lol. I should give it a try. I haven't been doing leg exercises, but just like you said it's going to be hard the first months. Thanks for the info now I know how not to hurt myself lol.