Understanding Avogrado's hypothesis
If we consider **Ideal gas equation **
and ignore the type of interaction which are present in the real gases then ; 
$PV=nRT $
wherevP= pressure ; v=vol. ; n= moles; R = universal gas constant ; T= temp.

$PV/RT=constant $
so $n=constant$

so if$ n$ is constant then if we multiply it with avogadro's constant $N_A $ it would give you number of molecules .
Again that's when ideal gas .