do you pronounce the singular and plural of Reese’s the same?
@Lady@glitch.cat.family Yes, because it would be a shortened form of "Reese's piece" (I assume we're talking about the candy?).
(That would also apply more broadly to other Reese's products. It would only be "Reese'ses" if you were trying to refer to multiple companies called Reese's. And even then I'd probably say something like "Reesei" because "e'ses" is a mouthful... 
)
@alis where i live in the States, Reese’s is the usual short form of “Reese’s Peanut Butter Cup”, not “Reese’s Pieces” (which is a less common candy, always said in full)
but i realize that this may not be universal, and the question is ambiguous enough to allow any interpretation! (the spirit of your response holds regardless; it’s just, i imagine, much less common to refer to a single Reese’s Piece than a single peanut butter cup.)
@Lady@glitch.cat.family Neither of them are common here and writing the response actually gave me a panic because I'd referenced the cups recently in a fic and was like "... wait did I call them pieces by accident???" 
But hence the note expanding it to other Reece's-produced goods, lol.