What is the proper procedure for Birkat Kohanim (the priestly blessing) if the congregation consists entirely of Kohanim? Do all the worshippers ascend to recite the Beracha, or do only some of them recite the Beracha?
The Halacha in this case depends on how many people are present in the synagogue. If the Minyan is comprised of only ten Kohanim and nobody else, then they all ascend for the recitation of Birkat Kohanim. Women and children present at the synagogue would answer "Amen" to the Beracha. If there are no women and children in the synagogue, then of course nobody would answer "Amen," but this does not undermine the validity of the Birkat Kohanim.
This situation, of course, gives rise to the interesting question of whom they are blessing, since nobody is present in the synagogue to receive their blessing. The answer is that they bless all other Jews, who are not in attendance in the synagogue. The Kohanim customarily wave their hands side to side at certain points during the recitation of Birkat Kohanim, in effect bestowing the blessing to those beyond the synagogue walls.
If more than ten Kohanim are present (but no non-Kohanim are present), then all the Kohanim go up except 10, to maintain a Minyan.
In all cases, the Chazan (even though he is a Kohen) does not participate in Birkat Kohanim, and instead remains in his place and dictates the Beracha to the other Kohanim.
Summary: If a Minyan consists of only ten people, all of whom are Kohanim, they all recite Birkat Kohanim, except for the Chazan, who stays in his place and dictates the Beracha to the others. If the Minyan consists of more than ten Kohanim but nobody else, all the Kohanim go up, so long as at least 10 stay down.