Charles Ellicott Commentary Numbers 10:4

"And if they blow but one, then the princes, the heads of the thousands of Israel, shall gather themselves unto thee." — Numbers 10:4 (ASV)

With one trumpet. —Or, but once (or, at the same time). (Proverbs 28:18; Jeremiah 10:8.) Some suppose that the meaning is that the trumpets were to be blown at the same time with one even or uniform sound, and that not a continuous one.