Prawns are sold on a scale of number of prawns per pound which is determined by their size. 14-16 prawns means there are 14 to 16 prawns per pound and are fairly large. 16-20, 20-30 are other sizes. Most prawns cook quickly 3-5 minutes, and turn whitish in color. They should also be added last as they do cook quickly and will get tough if they cook a long time.
Here's my suggestion: take a little of the sauce, about 3/4 cup and put it in a saute pan and bring it to just under bubbling. Add the prawns and cook until the prawn turns white about 3-4 minutes, maybe longer if they are larger. Then add everything back into the nain sauce pan. Make sure you have taken the shells off also. You may also want to butterfly the prawns by making a cut along the back, from back to front and then spreading the two halves which is still connected at the tail or "butterflying" it.