In Act IV, Scene 1, Macbeth visits the witches again for advice. They first show him three apparitions - an "armed head", a "bloody child", and a "child crowned, with a tree in his hand." Each warns/advises Macbeth of who to fear and not to fear, and what signs to watch for. After this, the witches show eight kings, all of whom resemble Banquo, the point being that the line of Kings will come from Banquo's line (not Macbeth's). The last king is James I (the king at the time of Shakespeare's writing of the play), and it is suggested that the line will continue beyond James (which would have pleased the monarch watching the play very much). None of this was good news for Macbeth!
Many believe that Shakespeare had pleasing King James I of England in mind while writing "Macbeth." James was very interested in witchcraft, which plays a large role in the tragedy. There are also numerous references to a benevolent English king while Malcolm and Macduff are in England. Therefore, many view the showing of the 8 kings to also be flattering towards James I. It is thought that the 8th king, the one holding the glass, or mirror, is meant to represent James, signifying that his descendants will continue to rule. Many also point to the reference to the "two-fold balls" and "treble-sceptered" references, possibly suggesting that Shakespeare thought descendants of James I would rule a united England and Ireland ("two-fold") or a united England, Ireland, and Scotland ("treble").
As far as "naming" the 8 kings, there is no suggestion in the text of the play that the kings have names or are meant to.
When In Act IV Sc.1 Macbeth meets the witches he demands that they answer his question: "Shall Banquo's issue ever/Reign in this kingdom?" Initially the witches refuse but just before they vanish they remark, "Show his eyes and grieve his heart." immediately, "Eight kings appear, and pass over in order,the last with a glass in his hand; with Banquo following."
Shakespeare does not name the eight kings, because historically not much is known about Banquo, in fact some historians even doubt his very existence! However in Shakespeare's own time King James I of England was believed to have been a direct descendant of Banquo. Macbeth's speech, "Thou art too like the spirit of Banquo.....And points at them for his" is thus meant to flatter the new Stuart King James I.
After Queen Elizabeth died childless, King James VI of Scotland a Stuart was crowned the King of England as King James I. Two coronations were held, one in Scotland where one scepter was used and one in England where two scepters were used: "That two-fold balls and treble scepters carry." Scholars also interpret "treble scepter" to mean that King James' coronation united the three kingdoms of Scotland, England and Ireland.
Thus, the eight kings would refer to all the monarchs of the Stuart dynasty who ruled England from 1603-1714. However,historically there were only six Stuart kings.