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.
سَمِعْتُ رَسُولَ اللَّهِ ﷺ يَقُولُ عَنِ الْغُلاَمِ شَاتَانِ مُكَافِئَتَانِ وَعَنِ الْجَارِيَةِ شَاةٌ