In Vayikra 22:32 it says "Venikdashti BETOCH bnei yisrael" -
I will be hallowed amongst the Jews.
The gemarra in Brachot 21b connects this with Bamidbar 16:21
"Hibadlu METOCH haedah haraa hazot" - separate yourselves from
this evil group.
The logic is as follows:
"Venikdashti" - "I will be hallowed" concerns three prayers -
Barchu, Kaddish, Kedushah. These must be said amongst
a group of Jews.
How many? When Hashem gave the Torah to Moshe, Hashem also
gave him the Oral Torah which included a number of
"word connections" like the one above - BETOCH/METOCH.
The pasuk in Bamidbar concerns the meraglim (spies sent by
Moshe to visit Eretz Yisrael) TEN of whom were evil. So
using the "word connection" of BETOCH/METOCH, we
know that "Venikdashti" needs 10.
Behatzlacha rabba,
Yisrael