"Ennead" in ancient Greek means a collection of nine things.
Under the name Enneads Porphyrius published the work of his teacher Plotinus,
= Εννεάδες, in ancient Greek.
It is a corpus of six books of nine treatises each, that is:
The number three is also symbolic. The neoplatonic hypostases are three: One - Intelligence - Soul. The nine is a multiple of three.
Finally, the six Enneads x nine treatises each = 54 = 5+4=9, nine = Ennead (Εννεάς).
As Iamblichus in the Theology of Arithmetic says: that number admits nothing beyond the ennead, but everything circles around within it, is clear from the so-called recurrences: there is natural progression up to it, but after it there is repetition.