Sanhedrin 1:1

The following kinds of legal cases are judged by a court of three judges: financial cases; cases of theft and injury; cases of damages, half-damages, and double, four-fold and five-fold payments; cases of rape, seduction and defamation (referring to a groom who accuses his bride of infidelity); this is the opinion of Rabbi Meir. The Sages say that a defamation case is judged by a court of 23 because it involves the possibility of becoming a capital case (i.e., of adultery).

Sanhedrin 1:2

Lashes are administered by three judges. It was stated in Rabbi Yishmael’s name that this requires 23 judges. Rabbi Meir says that the new month and a leap year were determined by a court of three; Rabban Shimon ben Gamliel says that they start with three, they discuss it with five, and they decide with seven. However, if they decided with three, what they decided is effective.