Rabbi Moshe Cordovero on the Necessity of the Ten Sefirot

"It is incumbent upon the sefirot to be ten, neither nine nor eleven, since the upper number is the source of the sefirot, whereas its assemblage is their boundary and number."

-- Rabbi Moshe Cordovero, Eilimah Rabbati - Ein Kol 4:16 (transl. Z. Raviv in "Decoding the Dogma within the Enigma", 2007)