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.