The Spanish plural of quinceañera is quinceañeras.
Spanish normally forms this feminine noun’s plural by adding -s because the singular ends in an unstressed vowel. The result is quinceañeras, referring to multiple girls or young women associated with the celebration. The spelling retains the ñ and the accent mark on the á.
Quinceañero is a different masculine form that can refer to a boy’s comparable fifteenth-birthday celebration or, depending on context, a male participant. It should not be substituted for the feminine plural quinceañeras when discussing several female honorees.
English usage often borrows quinceañera as a loanword and may omit diacritics in informal writing. In standard Spanish spelling, however, the ñ and accent are part of the correctly written word.