How many priests sounded trumpets with the Levite singers at the temple's dedication?

Answer: 120

The Levite singers stood east of the altar in fine linen, with 120 priests sounding trumpets, as they praised the LORD together.

“all the Levitical singers—Asaph, Heman, Jeduthun, and their sons and relatives—stood on the east side of the altar, dressed in fine linen and playing cymbals, harps, and lyres, accompanied by 120 priests sounding trumpets.”— 2 Chronicles 5:12 (BSB)