Why is Bach’s St. Anne Fugue considered a triple fugue?

Johann Sebastian Bach’s Fugue in E-flat Major for organ, BWV 552, is known as the St. Anne fugue and is classified as a triple fugue because it features three distinct subjects.1

In this composition, each of the three subjects is introduced in its own separate exposition, with each exposition utilizing a different metre. While the work is a five-voice fugue, a full five-part texture is only present during certain passages, with the remainder of the piece alternating between two, three, or four parts. Throughout the work, the first subject is combined with each of the other two subjects, demonstrating the complex counterpoint characteristic of a triple fugue, where multiple subjects are treated both separately and simultaneously.12