Note: Contrapunctus 15 is known elsewhere in our catalog as Contrapunctus 14. Those collections that contain fifteen Contrapuncti or cite number 15 (such as this work) derive that number from an early keyboard edition that included an extra fugue - called No 14 - that is really a variant on No 10 (BWV.1080 nr 10). In actuality, Bach wrote only fourteen Contrapuncti*, with number 14 being the final, though incomplete, movement. Score & parts. -cdp
* Sixteen if you consider the Inversus versions of Nos. 12 and 13 as separate Contrapuncti. For sake of argument, I consider these to be part and parcel with their Rectus versions.