What is the plural of plenary?

Short answer: plenaries, but ...

"Plenary" is an adjective meaning complete. It is often used to describe a meeting to which everyone is invited or expected to attend. Thus, we talk about plenary sessions at a conference.

If you were to fall into the habit of referring to a plenary session as a plenary (and a break-out session as a "break-out") then, by extension, you would refer to several plenary sessions as plenaries. But why would you want to pass up a perfectly good opportunity to demonstrate that you know what "plenary" means, unless you were tweeting: "Attending both plenaries and 4 or 5 break-outs at today's #grammarconference!"

It isn't inherently wrong to use an adjective as a noun; we do so a lot; e.g., "Respect the old, for one day you will be old yourself." But we must guard against overdoing it.

That being said, "plenary" can also mean a small book that contains all four Gospels, so one could properly say, "The church gave plenaries to the initiates." At one time, these books were probably referred to as plenary collections and eventually, more simply, as plenaries.

Lesson: when adjectives and nouns are frequently paired together in a professional or vocational setting, the nouns are likely to be dropped and the adjectives to become nouns. Thus, we create jargon, and in doing so we risk losing sight of the original versatility of the adjectives. One might know that a plenary is a collection of the four Gospels without knowing why it is referred to as such, with no sense of the completeness of the object.