The volume of the H2 gas will depend on the temperature and the pressure of the gas. If we assume STP (standard temperature and pressure), and we recall that at STP 1 mole of an ideal gas = 22.4 liters....
N2 + 3H2 ==> 2NH3 ... balanced equation
moles H2 required = 35.5 g NH3 x 1 mol NH3/17 g x 3 mol H2 / 2 mol NH3 = 3.13 moles H2 needed
At STP, 3.13 moles H2 x 22.4 L/mol = 70.16 liters = 70.2 L H2 needed
(NOTE: at any temperature and pressure other than STP, the volume of H2 will be different)