I heard the Messenger of Allah ﷺ say: Two resembling sheep are to be sacrificed for a boy and one for a girl. AbuDawud said: I heard Ahmad (ibn Hanbal) say: The Arabic word mukafi'atani means equal (in age) or resembling each other. (Using translation from Abū Dāʾūd 2834)
رَسُولَ اللهِ ﷺ قَالَ عَنِ الْغُلَامِ شَاتَانِ مُكْفَأَتَانِ وَعَنِ الْجَارِيَةِ شَاةٌ