I've been training legs with Leroy Davis for the last five or six weeks and we work up to 11 plates a side on the leg press. Proper form, slow controlled reps, bringing the knees right back to the chest - feet are turned slightly outwards to take the pressure off the knees. Three sets to failure. In the past when training with a former training partner I've done 15 plates a side but to be honest they were more like half or three quarter reps - it's so much better to drop the weight and crank them out properly.