Wa alaykum salam wa rahmatuLlahi wa barakatuHu,
According to the Shafi’i Madhhab, an unmarriageable relationship by suckling is established by five instances of breastfeeding. And marriage is permissible when one knows there was less than that. Still, it is prudent to avoid. Imams Abu Hanifah and Malik said that it is established by one; and to avoid practicing on this difference of opinion one should avoid the marriage. (Tuhfat al-Muhtaj w/ Hawashy Ibn Qasim 8/288; Rawdah 9/9; Fatawa Ibn al-Salah 2/451)
And Allah knows best.
Answered by: Shaykh Yaqub Abdurrahman