For the source text click/tap here: Sanhedrin 13
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The rabbis then consider how many judges are required to grant smicha to other judges. It was understood that three judges were needed, but the rabbis cannot find a proper proof text for this custom. In their search for the origin of this practice, the rabbis find proofs for other possible numbers of judges.
According to the Mishna, among the ceremonies that require the participation of three judges is semikhat zekenim – rabbinic ordination.
In searching for a source for this requirement, Abaye points out a difficulty – if the source is the passage (Bamidbar 27:23) where Moshe lays his hands on Yehoshua to declare him his successor, then it would appear that a single judge would suffice. And if we saw that Moshe embodies the Sanhedrin and is considered the equivalent of its 71 members, then we should need a full Sanhedrin to confer rabbinic ordination.
We explore the ordination process then and now.