This is a style of speech. Sometime the speaker says I and sometime says we. We also use that in our conversations. In the Quran you will see that often the first person singular such as I or My is used, when Allah speaks about His love, care and closeness and forgiveness for His servants.
In a similar way the first person plural is often used when Allah speaks about His power, majesty, glory, great deeds or when He speaks about His anger and wrath for the sinners and criminals.
See for example the verses where the first person singular is used:
When My servants ask thee concerning Me, I am indeed close (to them): I listen to the prayer of every suppliant when he calleth on Me: let them also, with a will, listen to My call, and believe in Me: that they may walk in the right way. (al-Baqarah 2:186)
Or Verily, I am Allah: there is no god but I: so serve thou Me (only), and establish regular prayer for celebrating My praise. (Taha 20:14)
or But, without doubt, I am (also) He that forgives again and again, to those who repent, believe, and do right, who, in fine, are ready to receive true guidance. (Taha 20:82)
or see another example where both pronouns are used side by side,
Before them the People of Noah rejected (their Messenger): they rejected Our servant, and said, Here is one possessed! and he was driven out. Then he called on His Lord: I am one overcome: do Thou then help (me)! So We opened the gates of heaven, with water pouring forth. And We caused the earth to gush forth with springs. So the waters met (and rose) to the extent decreed. But We bore him on an (Ark) made of broad planks and caulked with palm-fiber: She floats under Our eyes (and care): a recompense to one who had been rejected (with scorn)! And We have left this as a Sign (for all time): then is there any that will receive admonition? But how (terrible) was My Penalty and My Warning? (al-Qamar 54:9-16)
after all 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.