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 Qur'an, where Allaah addresses the Arabs in their own tongue. ( Fataawa al-Lajnah al-Daa'imah, 4/143).
"Allaah, 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" [al-Fath 48:1], and other similar phrases. But Allaah 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).
These words, innaa ("Verily We") and nahnu ("We"), and other forms of the plural, may be used by one person speaking on behalf of a group, or they may be used by one person for purposes of respect or glorification, as is done by some monarchs when they issue statements or decrees in which they say " We have decided…" etc. [This is known in English as "The Royal We" – Translator]. In such cases, only one person is speaking but the plural is used for respect. The One Who is more deserving of respect than any other is Allaah, may He be glorified and exalted, so when He says in the Qur'an innaa ("Verily We") and nahnu ("We"), it is for respect and glorification, not to indicate plurality of numbers. If an aayah of this type is causing confusion, it is essential to refer to the clear, unambiguous aayaat for clarification, and if a Christian, for example, insists on taking ayaat such as
"Verily, We: it is We Who have sent down the Dhikr (i.e., the Qur'an)"
[al-Hijr 15:9 – interpretation of the meaning] as proof of divine plurality, we may refute this claim by quoting such clear and unambiguous aayaat as (interpretation of the meanings):
"And your god is One God, there is none who has the right to be worshipped but He, the Most Beneficent, the Most Merciful" [al-Baqarah 2:163]
"Say: He is Allaah, the One" [al-Ikhlaas 112:1]
and other aayaat which can only be interpreted in one way. Thus confusion will be dispelled for the one who is seeking the truth. Every time Allaah uses the plural to refer to Himself, it is based on the respect and honour that He deserves, and on the great number of His names and attributes, and on the great number of His troops and angels.
(Al-'Aqeedah al-Tadmuriyyah by Shaykh al-Islam Ibn Taymiyah, p. 109).