If im 5"6 and weigh 128 pounds how do you know how fast your metabolism is and how manny meals you should eat a day along with portion size?

In my opinion you --: Your weight is ideal for your age and height. As long as you maintain it, you'll be doing good. You can experiment with smaller more frequent meals and assess the result. Eating healthy with regular exercises to keep your heart rate between 70-80 after 20-30 minutes, 4-5 days a week, would be great. Check with your pcp, as he/she knows you best.