Jude 14 The word for ten thousand, is representation of a tremendous number- myriads, it has the meaning of the highest number at that time that one could calculate.

Creatures located in Heaven are described in Dan 7:10 as “A fiery stream issued and came forth from before Him. A thousand thousands ministered to Him; ten thousand times ten thousand stood before Him. The court was seated, and the books were opened.”

Rev 5:11 “Then I looked, and I heard the voice of many angels around the throne, the living creatures, and the elders; and the number of them was ten thousand times ten thousand, and thousands of thousands. ” The bible teaches that heaven has a vast population of both angels and people- saints. These are those (either one or both groups) who will come with him when he comes to earth to judge and set up his kingdom.

Jude was writing under the inspiration of the Holy Spirit (2 Peter 1:20-21), we know that what he quoted from Enoch was true. This is confirmed by the fact that the same idea about the Lord returning with His holy ones to render judgment found elsewhere in the Bible, Zechariah 14:5, Isaiah 66:15, and Psalm 96:13, Deut. 33:2.

The apocryphal writing called "the Book of Enoch," contains a statement resembling Jude’s, but there is no proof it existed at the time of Jude.

Numerous apocryphal works were composed at the same time period with the Apocalypse-such as the Book of Enoch, the Ascension of Isaiah, the Testament of the Twelve Patriarchs, the Sibylline Oracles, the fourth Book of Ezra, the Pastor of Hermas, and others.

Whether his quotation is derived from tradition or from Enoch’s writing is uncertain. Justin Martyr, Irenaeus, Clement of Alexandria, and others mention the Book of Enoch.  “The Apostolic Constitutions, Origen (contra Celsus), Jerome, and Augustine deny its canonicity” (Fausset's Bible Dictionary). [the Early church tradition favored Enoch and Elijah as the identity of the "two witnesses" in Rev. 11:3.]

“The Book of Enoch, which was known to the fathers of the second century, was lost for some centuries with the exception of a few fragments, and was found entire in a copy of the Ethiopic Bible, in 1773, by Bruce. It became known to modern students through a translation from this into English by Dr. Lawrence, in 1821. It was probably written in Hebrew. It consists of revelations purporting to have been given to Enoch and Noah, and its object is to vindicate the ways of divine providence, to set forth the retribution reserved for sinners, angelic or human, and "to repeat in every form the great principle that the world-natural, moral, and spiritual-is under the immediate government of God." (Vincent's Word Studies of the New Testament)

This book was discovered in an AEthiopic version. Bruce the Abyssinian traveler brought home three Ethiopic copies from Alexandria, which Lawrence translated in 1821. The Ethiopic was translated from the Greek, the Greek from the Hebrew. and was published with a translation by Dr. Laurence of Oxford, in 1821, and republished in 1832. A full account of it and its contents may be seen in an article by Prof. Stuart in the Bib. Repository for January 1840, pp. 86-137.

“The Slavonic Enoch. In the year 1892 the attention of Dr. Charles was directed to the fact that a Book of Enoch was extant in Slavonic. Perusal proved it not to be a version of the book before us, but another and later pseudepigraphic book, taking, as the earlier had done, the name of Enoch. It is totally independent of the Ethiopic Enoch Book, as is seen by the most cursory consideration. It begins by giving an account of Enoch's instruction to his descendants how he had been taken up to the seventh heaven. Another manuscript adds other three heavens. In the third (?) heaven Enoch is shown the place of the punishment of the wicked. In the description of the fourth heaven there is an account of the physical conditions of the universe, in which the year is said to be 365 1/4 days; but the course of the sun is stated as a course of 227 days; which appears to be all that is accounted for.

Here the independence of the Slavonic Enoch is clear, as the Ethiopic Enoch makes the year 364 days. There are many points of resemblance which show that the writer of the Slavonic Enoch had before him the book which has come down to us in Ethiopic, but the relationship is not by any means so close as to be called dependence.” (from International Standard Bible Encyclopaedia)

The true prophecy of Enoch, though unrecorded, could have been handed down by tradition, as the Jews had a meticulous way of keeping both written and oral tradition. Paul mentions Jannes and Jambres the Egyptian magicians, names known in Jewish tradition, not from Scripture (2 Tim. 3:8). For him to do this and be accurate God would have had to confirm the tradition.

Even if Jude cites a passage from this non-canonical book, it does not mean he accepted the whole book as true, only this particular statement. I think it is more likely Jude did not lift this statement from the non-Biblical book of Enoch,. It was either something passed on orally or he received it as a direct revelation from God.