Numerical answer type, tolerance 0.5.
Answer: 2
Butane-2,3-diol drawn is the meso (2R,3S) isomer, so its two methyl-bearing ring carbons keep configurations that put the methyls cis in the ring. The new acetal carbon C2 carries phenyl, hydrogen and two ring oxygens, and phenyl can be cis or trans to the methyls. RDKit enumeration with the diol centres fixed gives 2 distinct stereoisomers (both achiral, diastereomers).