Pollux, Vocabulary (Onomasticon) 4.110

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τὸ δὲ παλαιὸν ὁ τραγικὸς χόρος πεντήκοντα ἦσαν, ἄχρι τῶν Εὐμενίδων Αἰσχύλου· πρὸς δὲ τὸν ὄχλον αὐτῶν τοῦ πλήθους ἐκπτοηθέντος συνέστειλεν ὁ νόμος εἰς ἐλάττω ἀριθμὸν τὸν χορόν.

Formerly, the tragic chorus had fifty members, until the Eumenides of Aeschylus. For when the audience was scared out of their wits by their multitude, a law reduced the number of chorus members.

Relevant guides	Aeschylus