Albert Barnes Commentary Numbers 10:8

"And the sons of Aaron, the priests, shall blow the trumpets; and they shall be to you for a statute for ever throughout your generations." — Numbers 10:8 (ASV)

The sons of Aaron - As the trumpets were emblematic of the voice of God, the priests only were to use them. At this time there were only two “sons of Aaron;” but in later times, when the number of priests was greater, more trumpets were used; we read of seven in the times of Joshua and David (see the marginal references); and of a hundred and twenty in that of Solomon (2 Chronicles 5:12).