Among thousands of human beings, perhaps one strives for perfection; among those who have achieved perfection, scarcely one knows Me in truth.

Hierarchy of rarity: of thousands of manuṣyā (humans), one (kaścit) yatati (strives) for siddhi (spiritual realization). Of these accomplished ones (siddhānām), hardly one vetti tattvataḥ (knows in reality/essence).

Tattvataḥ — not superficially, not theoretically, but in essence. Yoga is common; knowing Kṛṣṇa as He is (saccidānanda, bhagavān) is exceptional.