  A single atomic slice of {\alpha}-tin-stanene-has been predicted to host
quantum spin Hall effect at room temperature, offering an ideal platform to
study low-dimensional and topological physics. While recent research has
intensively focused on monolayer stanene, the quantum size effect in few-layer
stanene could profoundly change material properties, but remains unexplored. By
exploring the layer degree of freedom, we unexpectedly discover
superconductivity in few-layer stanene down to a bilayer grown on PbTe, while
bulk {\alpha}-tin is not superconductive. Through substrate engineering, we
further realize a transition from a single-band to a two-band superconductor
with a doubling of the transition temperature. In-situ angle resolved
photoemission spectroscopy (ARPES) together with first-principles calculations
elucidate the corresponding band structure. Interestingly, the theory also
indicates the existence of a topologically nontrivial band. Our experimental
findings open up novel strategies for constructing two-dimensional topological
superconductors.
