Manu smriti 3.125. One must feed two (Brahmanas) at the offering to
the gods, and three at the offering to the manes, or one only on
either occasion; even a very wealthy man shall not be anxious (to
entertain) a large company
Prescribed in all the Smritis, that deal with Vedic rituals.
Manu Smriti further states:
3.144. (If no learned Brahmana ...