So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?
Have you tried to calculate?
Why does it need a calculation to confirm? If you we prepare the 5 mM ammonium phosphate buffer by adding salt to have 5 mM, then adjusting the pH with phosphoric acid, isn't it very obvious that if some of the phosphoric acid will be included as phosphate as it is clear that phosphoric acid is phosphate as well?
My original question in the post was:
"If there is instruction that 'prepare a 5.0 mM phosphate buffer solution by adding 5mM of ammonium phosphate monobasic and then adjust the pH to x.x with H3PO4 solution', can we still call this solution a 5.0 mM phosphate buffer? My concern is that we don't know how much H3PO4 solution is added. It doesn't matter for molarity calculation?"
Based on what you said, it seems obvious that the adjusting the pH with H3PO4 will increase the amount of phosphate in the buffer which has already 5mM of ammonium phosphate monobasic, so the molarity of phosphate will increase and it can't stay the same or decrease.
It is the question doesn't need the calculation. This is the yes or no question. Why do I need to do the calculation?