Goal of this paper is to study the following singularly perturbed nonlinear Schrödinger equation: eps^2s (-Delta)^s v + V(x)v = f(v), x in R^N, where s is in (0,1), N is greater or equal to 2, V in C(R^N,R) is a positive potential and f is assumed critical and satisfying general Berestycki-Lions type conditions. When eps is greater than 0 is small, we obtain existence and multiplicity of semiclassical solutions, relating the number of solutions to the cup-length of a set of local minima of V; in particular, we improve the result in [37]. Furthermore, these solutions are proved to concentrate in the potential well, exhibiting a polynomial decay. Finally, we prove the previous results also in the limiting local setting s = 1 and N greater or equal to 3, with an exponential decay of the solutions
