Krishna states a plain fact. Among thousands of people, perhaps one makes a genuine effort toward spiritual realization. And among those who do strive, perhaps one actually comes to know Him in truth.
This is not meant to discourage. It is an honest acknowledgment of how rare the pursuit is. Think of a thousand children playing in a field. Among them, maybe one sits down to study with real focus. And among the few who study, perhaps one grasps the subject so deeply that it changes how they see everything. That rarity is what makes the pursuit precious.
Rather than creating despair, this shloka is meant to stir curiosity. If this knowledge is so rare, it must be extraordinary. And Krishna is about to open that very door in the shlokas that follow.