Where a Levite Torah scholar is present among other Levites, Rabbi Sternbuch writes that the other Levites should honor him by allowing him to wash the Kohanim’s hands alone. Moreover, he casts doubt concerning the general worthiness of the custom whereby several Levites join together (holding the washing-cup together) in washing a Kohen’s hands, stating that it is preferable for a single Levite to wash a Kohen’s hands.
However, the common custom, in particular among Ashkenazi communities, is for several Levites to hold the washing-cup together. It is possible that this custom emerged from the situation of Ashkenazi communities outside Israel, where Birkas Kohanim is only conducted on festival days, and all the Levites wanted to join in infrequent mitzvah of washing the Kohanim’s hands. Indeed, the Leket Kemach HaChadash (128:45) writes that the custom of several Levites washing the hands of a single Kohen is only practiced among Ashkenazi communities (he notes that for certain Ashkenazi communities, such as Frankfurt, a particular Levite family was appointed for washing hands.)
Yet, it appears that the common custom, among both Ashkenazi and Sephardic communities, is for several Levites to wash the hands of one Kohen.
