The Talmud explains:
As a mystical concept, the number 36 is intriguing.
It is said at all times there are 36 special people in the world. Were it not for them, all of them, if even one of them was missing, the world would come to an end.
The two Hebrew letters for 36 are the lamed, which is 30, and the vav, which is 6.
Therefore, these 36 are referred to as the Lamed-Vav Tzadikim.
This widely held belief is based on a Talmudic statement to the effect that in every generation 36 righteous “greet the Shechinah,” the Divine Presence.
–Tractate Sanhedrin 97b
Note: Schechinah denotes the dwelling or settling of the Divine Presence of God.