Pretty much what the title says. Where in the Torah do we find something for which we need 10 Kohanim or 1 Yisrael?
In a minyan with 11 Jews, if they are all Kohanim, one goes up to duchan and 10 stay behind and answer Amen. If one of them is a Yisrael, all the Kohanim can duchan and the one Yisrael answers Amen (he is significant on his own since the main mitzva is to bless the Yisraelim. (SA OC 128:2,5 MB 101)