πλήθει, dual use of πλῆθος in Philebus

Question

One word seems to me somehow very strange with its dual number. It is in a passage from Plato's Philebus:

φωνὴ μὲν ἡμῖν ἐστί που μία διὰ τοῦ στόματος ἰοῦσα, καὶ ἄπειρος αὖ πλήθει, πάντων τε καὶ ἑκάστου.

with a translation:

Sound, which passes out through the mouth of each and all of us, is one, and yet again it is infinite in number.

The word πλήθει is, as far as I've checked, a dual word of πλῆθος, meaning "a great number". I would think that the word shall be used as either singular or plural. Is there a reason that this word is especially dual?

Answer 1

It isn't dual; it's dative singular. The dative is a dative of respect and depends on the adjective ἄπειρος. Therefore, the phrase ἄπειρος...πλήθει means 'limitless in (with respect to) magnitude/extent/quantity.'

Answer 2

In the case you mention, πλήθει is actually the dative singular of πλῆθος. This is the standard declension for neuter 3rd declension nouns ending in vowel + sigma, as indicated on this page with the model of "τέλος."

It is a dative of respect modifying ἄπειρος, as the translation accurately captures: "infinite in number."