TT #81 · Parshas Tazria · Aliyah 1 · Passuk 3
English: The priest shall examine the affection on the skin: if hair in the affected patch has turned white and the affection appears to be deeper than the skin, it is a leprous affection on the skin; and if the priest sees that it is a leprous affection, the priest shall pronounce the person impure.
Chabad.org — Chumash, English translation + Rashi for Vayikra 13:3
Torah Temimah cites Sifra (Toras Kohanim) for the minimum number of hairs required for this sign.
Sifra — Vayikra 13, Midrash Sefer
English: “And hair in the lesion”—the minimum number of hairs is two.
Textual qualification
The Sifra derives that two hairs are the minimum number relevant to this sign.