The usual Spanish plural of quinceañera, when referring to female celebrants, is quinceañeras.
Quinceañera is a feminine Spanish noun formed from quince, meaning fifteen, and años, meaning years. In ordinary Spanish usage, its plural is made by adding s while retaining the ñ: quinceañeras.
The word can refer to a young woman celebrating her 15th birthday or, by extension, to the celebration itself. Context determines which meaning is intended. A plural reference to several celebrations can therefore require more wording to avoid confusion between people and events.
Quinceañero is commonly used for a male celebrant, while quinceañeros can be plural depending on context. That masculine counterpart should not be substituted when specifically referring to multiple girls or young women. Spanish spelling also matters: the ñ is part of the conventional form.