Your limited by what the manuf. make for sprockets, if it was me I wouldn't go more than 1 and if I needed more I would put a bigger sprocket on the back. If you go to few teeth the sprocket won't last as long, and a smaller sprocket has more resistance, one of the things to do on a race bike is go bigger on the front and rear to minimize the resistance. Not sure about the stress on the tranny being more with a small sprocket, that could be debateable, small sprocket=less clutching, bigger sprocket=more high rpm clutching to get out of corners.
Why do you ask the question? maybe that would help answer your question. 125's don't have a ton of bottom anyway, so no matter what your probably going to be fanning the clutch to accelerate out of corners.