If you have 9 moles of hydrogen gas(H2) the moles of water that can be formed is 9 moles
calculation
2H2 +O2→ 2H2O
The mole of H2O is calculated using the mole ratio.
that is H2:H2O is 2:2 into simple form = 1:1
therefore if 9 moles of H2 reacted the moles of H2O = 9 x1/1 = 9 moles