What you did so far doesn't make much sense. You need to convert number of molecules of ammonia to number of moles, then follow as UG suggested.
I don't think you even need to convert to number of moles. n[NH3]=molecules[NH3]/N
A, n[H2]=(stoichiometric factor)*n[NH3], molecules[H2]=n[H2]*N
A so molecules[H2]=(stoichiometric factor)*n[NH3]*N
A=(stoichiometric factor)*molecules[NH3]*N
A/N
A=(stoichiometric factor)*molecules[NH3].
Mostly unnecessary algebra but I wanted to prove that for myself. (stoichiometric factor) has been kept in this form so as not to reveal the final solution to OP.
Therefore the only thing we need to do is balance the equation, N
A cancels out. This is pretty intuitively obvious too.