Supernumerary is an adjective that describes something there’s too much of. If a scheduling mishap results in three extra people showing up for work, then you have supernumerary staff.
You probably know that something that's super is extreme in some way. And in supernumerary you’ll see part of the word “numeral,” which means “number.” So it makes sense that the adjective supernumerary describes an excess of something — an extraordinary number of that thing. Similarly, the noun version of supernumerary describes someone who's extra and has nothing to do. She just isn’t needed, like the player who never gets off the bench.