The usual Spanish plural of quinceañera is quinceañeras.
Spanish forms the plural by adding -s to a word ending in an unstressed vowel, while retaining the ñ and the accent mark on the á. Thus, one quinceañera becomes several quinceañeras. The word can refer to the celebrant, the party, or the broader event depending on context.
Quinceañero is a related masculine term, but it is not the plural of quinceañera. It may describe a boy’s comparable 15th-birthday celebration or, in some usage, a male participant, so the words should not be treated as interchangeable.
English-language writing often uses quinceañera unchanged as a borrowed singular noun and may use “quinceañeras” for the plural. Regional usage varies, but the Spanish spelling remains recognizable because the ñ distinguishes the word from forms without that letter.