The concept to be applied is Avagadro's law
The volume of a gas is directly related to the moles of the gas
or
at STP , one mole of a gas occupies 22.4 L of volume
the reaction of ammonia with oxygen is
4NH₃(g) + 5O₂(g) ---> 4NO(g) + 6H₂O(l)
as per equation four moles of ammonia will give four moles of NO
Or we can say that 22.4 X4 L of ammonia will give 22.4 X 4 L of NO
Hence
1000L of gas will give 1000L of NO gas