In the hadith of Sayyiduna Abu Bakr Siddiq (may Allah be pleased with him), it is stated that when the grazing goats are from forty to one hundred and twenty, then one goat is due as zakat. I have already written this hadith before.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب صدقة المواشي من الإبل والبقر والغنم / 2265