  We study the electronic structures and topological properties of
$(M+N)$-layer twisted graphene systems. We consider the generic situation that
$N$-layer graphene is placed on top of the other $M$-layer graphene, and is
twisted with respect to each other by an angle $\theta$. In such twisted
multilayer graphene (TMG) systems, we find that there exists two low-energy
flat bands for each valley emerging from the interface between the $M$ layers
and the $N$ layers. These two low-energy bands in the TMG system possess valley
Chern numbers that are dependent on both the number of layers and the stacking
chiralities. In particular, when the stacking chiralities of the $M$ layers and
$N$ layers are opposite, the total Chern number of the two low-energy bands for
each valley equals to $\pm(M+N-2)$ (per spin). If the stacking chiralities of
the $M$ layers and the $N$ layers are the same, then the total Chern number of
the two low-energy bands for each valley is $\pm(M-N)$ (per spin). The valley
Chern numbers of the low-energy bands are associated with large,
valley-contrasting orbital magnetizations, suggesting the possible existence of
orbital ferromagnetism and anomalous Hall effect once the valley degeneracy is
lifted either externally by a weak magnetic field or internally by Coulomb
interaction through spontaneous symmetry breaking.
