I voted zero for the reasons stated in a prior post (no alternating wave present = no phase). I also noted that the transformer can be a source for more than one phase for the reasons also stated in a prior post (using the technically-correct definition of phase, more than one phase can be present)
But for the record, I am not a proponent of changing the transformer label. It is a single-phase transformer.
A simple labeling rule could be: "Only voltage phases will be considered for labeling purposes, and multiple phases that can be paralleled or that are opposing pairs will be counted as one phase in total."
The reason for calling it a single-phase transformer was based on work done back in the 1860's and is the following:
Combining two available forces from a center-tapped transformer produces a larger resultant force in most circuit applications when the two forces are in-phase, in-series. These combined forces will act as one combined larger series force so we call it single-phase. A more descriptive label would be "series additive single-phase".
The "single-phase" label is not because it is a pure single-phase system (i.e. a two-wire source) but that the two smaller combined phases result in a larger single phase. Considering one phase to be the return of the other (possible since they are opposing pairs) equates the combination to a larger single-phase system, but it is still a system containing two smaller phases. Both systems can be produced by one center-tapped winding, in the same manner that the quadrature, or four-phase system, can be produced by two center-tapped windings.