How many harpsichords are too many? For J.S. Bach, the answer is four. His Concerto in A minor for Four Harpsichords and Orchestra (1730-1733) is actually a transcription of Vivaldi’s Concerto for Four Violins and String Orchestra. William S. Harvey’s art tenure at Nonesuch continues with a handful of harpsichordists going at it in period garb. The album also contains Bach’s concertos for 2 and 3 harpsichords.