BDAG lists μυρίος (innumerable/countless) and μύριοι (ten thousand) separately. The meaning innumerable/countless occurs in the plural in 1 Cor. 4:15 and 14:19. Likewise, the meaning ten thousand can occur in the singular with collective nouns (see Smyth 351). See χίλιοι in 1 Mc 4:1 and 10:70 as an example: χιλίαν ἵππον, a thousand horse. Per LSJ: in late Greek...acc. to Gramm., μυρίος (parox.) is the indefinite, μύριος (proparox.) the definite number...; but this distn. is not observed in codd.