Passage 10.20.2
Λοκροὺς δὲ τοὺς ὑπὸ τῷ ὄρει τῇ Κνήμιδι Ἡρόδοτος μὲν οὐχ ὑπήγαγεν ἐς ἀριθμόν, ἀλλʼ ἀφικέσθαι σφᾶς ἀπὸ πασῶν ἔφη τῶν πόλεων· τεκμήρασθαι δὲ ἀριθμὸν καὶ τούτων ἔστιν ἐγγύτατα τοῦ ἀληθοῦς· ἐς Μαραθῶνα γὰρ Ἀθηναῖοι σὺν ἡλικίᾳ τε τῇ ἀχρείῳ καὶ δούλοις ἐνακισχιλίων ἀφίκοντο οὐ πλείους, τὸ οὖν μάχιμον Λοκρῶν τὸ ἐς Θερμοπύλας ἐλθὸν οὐκ ἂν ὑπέρ γε ἑξακισχιλίους ἀριθμοίη τις. οὕτω γένοιτο ἂν ὁ σύμπας στρατὸς διακόσιοι καὶ χίλιοι καὶ μύριοι. φαίνονται δὲ οὐδὲ οὗτοι τὸν χρόνον πάντα ἐπὶ τῇ φρουρᾷ τῶν Πυλῶν καταμείναντες· πλὴν γὰρ Λακεδαιμονίων τε αὐτῶν καὶ Θεσπιέων καὶ Μυκηναίων προαπέλιπον τὸ πέρας τῆς μάχης οἱ λοιποί.
As for the Locrians who live beneath Mount Cnemis, Herodotus did not include them in his reckoning of numbers, but rather says merely that they came from all their cities. Yet an estimate of their number can be made that is very near to the truth. For the Athenians who marched to Marathon—including even the old men and slaves—did not exceed nine thousand. One could therefore suppose that the fighting force of Locrians who arrived at Thermopylae was not more than six thousand. Thus, the total army could amount to eleven thousand two hundred altogether. Yet it appears that even they did not remain guarding the pass continuously; rather, apart from the Lacedaemonians themselves and the Thespians and the Mycenaeans, the remainder had departed before the battle reached its conclusion.