Made this this evening. I used 10 crimini mushrooms as they were quite large 2-3" across. I did have a little left over filling but only enough to fill one more like sized mushroom. Instead of 15 minutes total I cooked them for about 22 minutes. I used hot Italian sausage. I was concerned about the salt in the parmigiano-reggiano so I only did 2 ounces instead of 3 in the mix. I used mozzarella to make up the last ounce. Still a little salty. I also added 1/2 cup of finely diced green bell pepper. I would definitely make this again.
