  In this paper we deal with the following nonlocal systems of fractional
Schr\"odinger equations \begin{equation*} \left\{ \begin{array}{ll}
\varepsilon^{2s} (-\Delta)^{s}u+V(x)u=Q_{u}(u, v)+\gamma H_{u}(u, v) &\mbox{ in
} \mathbb{R}^{N}\\ \varepsilon^{2s} (-\Delta)^{s}v+W(x)v=Q_{v}(u, v)+\gamma
H_{v}(u, v) &\mbox{ in } \mathbb{R}^{N} \\ u, v>0 &\mbox{ in } \mathbb{R}^{N},
\end{array} \right. \end{equation*} where $\varepsilon>0$, $s\in (0, 1)$,
$N>2s$, $(-\Delta)^{s}$ is the fractional Laplacian,
$V:\mathbb{R}^{N}\rightarrow \mathbb{R}$ and $W:\mathbb{R}^{N}\rightarrow
\mathbb{R}$ are continuous potentials, $Q$ is a homogeneous $C^{2}$-function
with subcritical growth, $\gamma\in \{0, 1\}$ and $H(u,
v)=\frac{2}{\alpha+\beta}|u|^{\alpha} |v|^{\beta}$ with $\alpha, \beta\geq 1$
such that $\alpha+\beta=2^{*}_{s}$. We investigate the subcritical case
$(\gamma=0)$ and the critical case $(\gamma=1)$, and using
Ljusternik-Schnirelmann theory, we relate the number of solutions with the
topology of the set where the potentials $V$ and $W$ attain their minimum
values.
