How many piercings is a girl allowed to have on the ears?

It is permissible for a female to pierce any part of her ears for the purpose of wearing jewellery, whether she has one or multiple piercings. However, if it becomes the hallmark or fashionable trait of a specific non-Muslim or sinful group of people, she should avoid doing so, in order to stay away from Tashabbuh (resemblance) with the Kuffar and Fussaaq (sinful).

Rasulullah Sallallahu Alayhi Wa Sallam is reported to have said:

“Whoever imitates a people is from amongst them.”

[Sunan Abi Dawud]

Mulla Ali Al-Qari Rahimahullah explains that this refers to one who makes himself similar to the Kuffar, the sinful people, or the pious people; in terms of his clothing and otherwise.