I donated the filets for a fish fry for our church a few months back. There was 80 people (men, women, and children) and we cooked right at 480 filets (5.1 filets each) with side dishes and had about 10 filets left. These were mostly big fish. Being all men, I would suggest about 6 filets each for 120-130 filets. But now you have to cook for 21 since I know you're cooking!!!!
Have fun and I hope I haven't steered you wrong.