Or HaChaim on Numbers 22:15/אור החיים על במדבר כ״ב:ט״ו

1 ויוסף עוד בלק שלח שרים, Balak sent additional princes, etc. This means that he sent both the original emissaries as well as additonal higher ranking ones. We deduce this from the word מאלה, "than these." Unless the original emissaries were present with the additional ones the word מאלה makes no sense. The Torah should then have written the word מהם, "than those." The reason that Balak sent the original emissaries along was so as to prevent Bileam from denying what these emissaries had reported in his name.

2The word ויוסף may also indicate that Balak sent a larger number of emissaries than the first time. We could not have deduced this if the Torah had only written: וישלח עוד שרים רבים. I would then have understood the word רבים to mean that he did not send fewer delegates than the first time.