  Goal of this paper is to study positive semiclassical solutions of the
nonlinear Schr\"odinger equation $$ \varepsilon^{2s}(- \Delta)^s u+ V(x) u=
f(u), \quad x \in \mathbb{R}^N,$$ where $s \in (0,1)$, $N \geq 2$, $V \in
C(\mathbb{R}^N,\mathbb{R})$ is a positive potential and $f$ is assumed critical
and satisfying general Berestycki-Lions type conditions. We obtain existence
and multiplicity for $\varepsilon>0$ small, where the number of solutions is
related to the cup-length of a set of local minima of $V$. Furthermore, these
solutions are proved to concentrate in the potential well, exhibiting a
polynomial decay. We highlight that these results are new also in the limiting
local setting $s=1$ and $N\geq 3$, with an exponential decay of the solutions.
