The usual Spanish plural of quinceañera is quinceañeras.
Spanish forms the plural by adding -s to a word ending in an unstressed vowel, so quinceañera becomes quinceañeras. The word can refer to several female celebrants or to several celebrations, depending on context. It preserves the ñ used in the singular form.
Quinceañeras is different from quinceañeros, which is the masculine plural or a mixed-gender plural in standard Spanish usage. That distinction matters when describing several celebrations or a group of celebrants. English-language writing sometimes leaves Spanish nouns unchanged, but the established Spanish plural adds the final s.
The spelling should also retain the accent mark on the final á. Removing the accent changes the written form and can obscure the word’s pronunciation.