The letter Ñ appears in quinceañera but not in quince.
Ñ, called eñe in Spanish, is the distinctive letter in the word quinceañera. The spelling separates quince, meaning “fifteen,” from the feminine noun-forming ending used in the name of the honoree or celebration. In standard Spanish writing, the tilde over the N is part of the letter, not optional decoration.
English-language sources sometimes replace ñ with n when keyboards or text systems lack Spanish characters, producing quinceanera. That form is understandable, but quinceañera preserves the original Spanish spelling. The letter also distinguishes words in Spanish, where ñ is a separate letter from n.
Pronunciation varies somewhat when Spanish words enter English, but the written form remains a useful clue to the word’s language and origin. The related expression quince años does not contain ñ because años is a different Spanish word.