There are 22 calories contained in an average medium-sized fresh tomato. The calorie amount goes down with the age of the vegetable as the proteins begin to degenerate.
I'm not sure exactly how many, but considering that ketchup with all of its syrups and preservatives only has 20 calories per serving, you should be safe with a tomato.
Tomatos are suppose to be good for you, so why would their calorie content be an issue? I would imagine that depending on how they are prepared, the calorie content would vary.