Allāh 
said
قُل لَّوْ كَانَ ٱلْبَحْرُ مِدَادًا لِّكَلِمَٰتِ رَبِّى لَنَفِدَ ٱلْبَحْرُ قَبْلَ أَن تَنفَدَ كَلِمَٰتُ رَبِّى وَلَوْ جِئْنَا بِمِثْلِهِۦ مَدَدًا
Say, “If the sea were ink for [writing] the words of my Lord, the sea would be exhausted before the words of my Lord were exhausted, even if We brought the like of it as a supplement.” [Surah al-Kahf 18:109]
Allāh says that even if those oceans were ink for the Words of Allāh, and all the trees were pens, the pens would be broken and the water of the sea would run dry, and the Words of Allāh would remain, for nothing can outlast them. For no one can comprehend the greatness of Allāh or praise Him as He deserves to be praised, except the One Who praises Himself.
Allāh also said
وَلَوْ أَنَّمَا فِى ٱلْأَرْضِ مِن شَجَرَةٍ أَقْلَٰمٌ وَٱلْبَحْرُ يَمُدُّهُۥ مِنۢ بَعْدِهِۦ سَبْعَةُ أَبْحُرٍ مَّا نَفِدَتْ كَلِمَٰتُ ٱللَّهِ ۗ إِنَّ ٱللَّهَ عَزِيزٌ حَكِيمٌ
And if whatever trees upon the earth were pens and the sea [was ink], replenished thereafter by seven [more] seas, the words of Allāh would not be exhausted. Indeed, Allāh is Exalted in Might and Wise. [Surah Luqmaan 31:27]
Allāh tells us of His might, pride, majesty, beautiful Names and sublime attributes, and His perfect Words which no one can encompass. No human being knows their essence or nature, or how many they are.
Meaning even if all the trees on earth were made into pens and the sea was made into ink, and topped up with seven more like it, and they were used to write the Words of Allāh showing His might, attributes and majesty, the pens would break and the ink would run dry, even if more were brought.
Prophet ﷺ said this while he ﷺ was in the state of prostration during prayer:
لَاا أُحْصِي ثَنَاءً عَلَيْكَ أَنْتَ كَمَا أَثْنَيْتَ عَلَى نَفْسِك
I cannot praise You enough; You are as You have praised yourself. [Sunan Abi Dawood, 879]
It is worth mentioning, the number seven is used to indicate a large amount, it is not to be taken literally or to be understood as referring to the seven oceans of the world, as was suggested by those who took this idea from Israelite stories, which we neither believe nor reject.
The words
بِمِثْلِهِ
like it
do not mean merely another one, but another like it and another and another and another, etc., because there is no limit to the signs and Words of Allāh.
إِنَّ اللَّهَ عَزِيزٌ حَكِيمٌ
Verily, Allāh is All-Mighty, All-Wise. [Surah al-Luqmaan 31:27]
means, He is All-Mighty and has subjugated all things to His will, so nothing can prevent what He wills, and none can oppose or put back His decision. He is All-Wise in His creation, commands, Words, actions, Laws and all His affairs.
Our Lord is as He says He is and He is beyond what we can say.
This article is based on tafseer of Surah al-Kahf 18:109 and Surah Luqmaan 31:27 of Imam Ibn Katheer. The translation is based on Abridged version of Tafseer Ibn Katheer by Darussalam Publications