There are two issues in this matter:

The first is that what triggers the prohibition is five sucklings or more. This is the sound (sahih) position in the Madhhab. This has been narrated from Aisha, Ibn Mas'ud, Ibn al-Zubayr, 'Ata', and Tawus. It is also the opinion of al-Shafi'i. There is a second narration from Ahmad (1) that even a little or a lot of suckling causes prohibition. This is narrated from Ali and Ibn Abbas, and it is the opinion of Sa'id ibn al-Musayyib, al-Hasan, Makhul, al-Zuhri, Qatadah, al-Hakam, Hammad, Malik, al-Awza'i, al-Thawri, al-Layth, and the people of opinion (Ashab al-Ra'y). Al-Layth claimed that the Muslims have reached a consensus that a little or a lot of suckling causes prohibition in infancy, just as that which breaks the fast of a fasting person. They argued using the saying of Allah the Almighty: "...your mothers who suckled you, and your sisters through suckling" (3), and his saying, peace be upon him: "That which is prohibited by lineage is prohibited by suckling." Furthermore, it is narrated from 'Uqbah ibn al-Harith that he married Umm Yahya, daughter of Abu Ihab. A black slave woman came and said: "I have suckled both of you." I mentioned that to the Prophet (peace and blessings of Allah be upon him), and he said: "How, when she has claimed that she has suckled both of you!" Agreed upon (4). [And because this is] (5) an act to which an eternal prohibition is attached, so the count is not considered in it, just like the prohibition of mothers-in-law, and li'an (imprecation) is not necessitated by it, because it is merely a claim. The third narration (6) is that the prohibition is not established except by three sucklings. This is the opinion of Abu Thawr, Abu 'Ubayd,