No Idea why Alberto called for Boltzmann constant. I think you were on the right track.

I missed being corrected by you the last 4 years

p*V = n * R * T (n = number of moles)

p*V= N * k * T (N=number of molecules)

Since the initial question is to get the number of molecules if you use the second equation you don`t have to divide the final result with Avogadro`s number.

The very first equation you posted is correct, you`ll get the number of moles if you put in the right conversion factor for the conversion of volume, but then do not forget to divide by avogadros number to get how many molecules there are+

Sorry if I caused any confusion, both paths lead to exactly the same result, though