How much of his possessions did Zacchaeus promise to give to the poor?

Answer: Half

Zacchaeus said, "Half of my possessions I give to the poor, and if I have cheated anyone, I will repay it fourfold."

“But Zacchaeus stood up and said to the Lord, “Look, Lord, half of my possessions I give to the poor, and if I have cheated anyone, I will repay it fourfold.””— Luke 19:8 (BSB)