SB 7.10.18
sri-bhagavan uvaca
trih-saptabhih pita putah
pitrbhih saha te ’nagha
yat sadho ’sya kule jato
bhavan vai kula-pavanah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The Supreme Personality of Godhead said: My dear Prahlada, O most pure, O great saintly person, your father has been purified, along with twenty-one forefathers in your family. Because you were born in this family, the entire dynasty has been purified.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The word trih-saptabhih means seven multiplied by three. In one’s family one can count back four or five generations — to one’s great-grandfather or even one’s great-grandfather’s father — but since the Lord mentions twenty-one forefathers, this indicates that the benediction expands to other families also. Before the present family in which one has taken birth, one must have been born in other families. Thus when a Vaisnava takes birth in a family, by the grace of the Lord he purifies not only that family but also the families of his previous births.