And there are also many other things which Jesus did, the which, if they should be written every one, I suppose that even the world itself could not contain the books that should be written. John 21:25
"With the use of the first person singular ("I suppose,") we are apparently back with the author of the book. He has before told us that he has made a selection from what is known about Jesus and has written simply that people may know that Jesus is the Christ, God’s Son, and that in this faith they may have life eternal (20:30-31). Now he comes back to the thought that there is much more that could have been written.
When he talks about the impossibility of the world’s containing all the books that would have to be written for a complete account, he fits into a pattern that we find in antiquity. Thus a saying of Rabbi Jochanan b. Zakkai (who dies somewhere around A.D. 80) survives:
"If all the sky were parchment, and all the trees were writing pens, and all the seas were ink there would not be enough to write down my wisdom which I have learned from my teachers; and yet I have had the pleasure of only as much of the wisdom of the wise as a fly, who plunges into the ocean, takes away."
Or we might notice some words of Philo, the great Jewish scholar: "Were [God] to choose to display his own riches, even the entire earth with the sea turned into dry land would not contain them."
We should not try to interpret this final statement of the Gospel by working out precisely how many books could be fitted into the world. John is not giving an exact number of books, but is speaking of the vast amount that would have to be put down if all that Jesus said and did and the meaning of his life and death and resurrection were to be recorded. "
(Morris, Leon. 2000. Reflections on the Gospel of John. Peabody, MA: Hendrickson Publishers., pgs 749-750)