The usual Spanish plural of “quinceañera” is “quinceañeras.”
Spanish nouns ending in the unstressed vowel -a commonly form their plural by adding -s, so quinceañera becomes quinceañeras. The ñ remains part of the spelling, and the written accent remains on the á: quinceañeras.
The term can refer to multiple female celebrants or to multiple celebrations, depending on context. It is different from quinceañero, which may refer to a male celebrant or, in some usage, the celebration associated with a boy turning fifteen.
English-language writing sometimes drops diacritics or uses approximate spellings, but the standard Spanish form keeps both the ñ and the accent mark. “Quinceañerases” is not the Spanish plural.