I am a beginner at calisthenics and was wondering if someone could tell me the most basic workouts, along with their workouts and rest routine. I was able to do about 6-7 chin ups, 15-17 push ups 30-40 crunches, 20-25 windshield wipers etc, in good form couple of weeks back, but now the reps have decreased. Is it because my muscles are too sore? If so, what do I need to do? I currently have a knee injury, so I'm looking for upper body strengthening alone.
I've always considered it good practice to do calisthenics every day, if being good at calisthenics is your goal. Generally speaking calisthenics are useful for improving cardiovascular endurance, and the best practice for that is doing them often. Maybe you break up which exercises you do which day, ie chinups and prone bridge one day, pushups leg raises another, and do one group every day.
I wrote a beginners guide to calisthenics on my site a couple of months ago.
(Edit to add the workout)
However if you can do chin-ups you're possibly a bit ahead of this. I do recommend you train for bridges though, they'll work your legs in a way that shouldn't put too much pressure on your knees.
As for rest periods, that depends on your goals and training plan. If you're training for strength over muscle, then keep your reps low and your sets high. This will allow you to train more often.
If you're training for muscle mass then make sure you have good rest periods to let your muscles recover. You should also train higher reps, 10-15 is a good amount, with only 2-3 sets.
Either way, unless you're looking to build stamina, I wouldn't go over 20 reps. Once you can do this amount then perform a harder exercise instead.