How many elders of the town did Boaz gather at the gate to sit as witnesses?

Answer: Ten

Boaz sat down at the town gate, called the nearer kinsman-redeemer over, and took ten of the city's elders to sit with them.

“Meanwhile, Boaz went to the gate and sat down there. Soon the kinsman-redeemer of whom he had spoken came along, and Boaz said, “Come over here, my friend, and sit down.” So he went over and sat down. Then Boaz took ten of the elders of the city and said, “Sit here,” and they did so.”— Ruth 4:1-2 (BSB)