“And if you are in doubt about what We have sent down upon Our Servant [Muhammad], then produce a surah the like thereof and call upon your witnesses other than Allah, if you should be truthful.”(The Quran 2:23).
It is a feature of literary style in Arabic that a person may refer to himself by the pronoun nahnu (we) for respect or glorification. He may also use the word ana (I), indicating one person, or the third person huwa (he). All three styles are used in the Quran, where Allah addresses the Arabs in their own tongue.
(Fataawa al-Lajnah al-Daa’imah, 4/143).
“Allah, may He be glorified and exalted, sometimes refers to Himself in the singular, by name or by use of a pronoun, and sometimes by use of the plural, as in the phrase (interpretation of the meaning):
‘Verily, We have given you a manifest victory”-(The Quran 48:1), and other similar phrases. But Allah never refers to Himself by use of the dual, because the plural refers to the respect that He deserves, and may refer to His names and attributes, whereas the dual refers to a specific number (and nothing else), and He is far above that.”
(Al-‘Aqeedah al-Tadmuriyyah by Shaykh al-Islam Ibn Taymiyah, p. 75).