If a series of three films is called a trilogy, what is the term for a series of six films?

I would guess a hexalogy, or something similar, as the other terms for numerical series use Greek prefixes:
https://en.wikipedia.org/wiki/Numerical_prefix

This is an interesting question. You can call a series of four films (or books) a tetrology or a quartet, a series of five a quintet or pentalogy (or maybe even quintology), but "sextet" seems to be the standard usage for a series of six. "Sextology" and "hextology" have apparently been used by some, but they do not appear to be widely-accepted words.

Sextet:

A sextet (or hexad) is a formation containing exactly six members. It is commonly associated with vocal ensembles (e.g. The King's Singers, Affabre Concinui) or musical instrument groups, but can be applied to any situation where six similar or related objects are considered a single unit.

https://en.m.wikipedia.org/wiki/Sextet