The plural form of sequel is sequels.

How do you pluralize a series?

Multiple Series (Plural)

And although series is a singular noun whose very meaning implies a plurality (“made a series of mistakes”), it can be pluralized just like any other count noun. However, the plural form of series is what is known to grammarians as a zero plural.

What comes after sequel?

A sequel to the first sequel might be referred to as a "third installment", a threequel, or a second sequel. A prequel is installment that is made following the original product but portrays events prior those of the original work.