The Spanish plural of quinceañera is quinceañeras.
The word quinceañera refers to a girl or young woman celebrating her fifteenth birthday, and its plural is formed by adding s while retaining the ñ. Thus, quinceañeras refers to multiple female honorees or multiple celebrations of this kind.
The spelling matters because quinceañera contains ñ, the Spanish letter pronounced differently from n. Quinceañero is a related masculine form used in some contexts for a boy’s fifteenth-birthday celebration, so it is not the correct plural for female quinceañeras.
In English, quinceañera is also commonly used as the name of the celebration itself. English usage may leave the word unchanged in some contexts, but the Spanish plural form remains quinceañeras. The term should not be confused with quince años, which literally means “fifteen years.”