Q: how many calories in one push up

A: Very few. You are doing yourself a disservice by approaching pushups with a 'burn calories' perspective. The main point of pushups and other weight training exercises is not to burn calories while doing them (though they will burn some), but to provide enough stress to physically damage your muscles which will in turn force your body to repair them afterwards which in turn burns far more calories and raises your metabolism.

So the goal of resistance training for weight loss is to provide sufficient stress to physically damage the fibers within your muscles. That requires, in general, using weights or resistance heavy enough that you can't do more than 12 repetitions in a row without.

So you'll most benefit from pushups when you are not yet very strong and can only do a few of them in a row. Once you can do 20+ pushups in a row they'll provide only limited stimulus to your muscles and become more like a traditional 'calorie-burning' exercise -- at that point you might as well just run or do some other form of cardio instead since you'll burn more calories through that than through pushups. And for resistance training, instead of doing dozens of pushups, you'd benefit more from heavy whole-body weight training.

So, to review, pushups burn very few calories while you actually do them, but if they are difficult for you to do (you can only do a dozen or less) then they will provide other fat-burning and metabolism benefits AFTER you do them.