In Act 4, Scene 1 of Macbeth, what does it mean when Macbeth says, "And yet the eighth appears, who bears a glass / Which shows me many more"?

In Act 4, Scene 1, Macbeth confronts the three witches and asks them questions about the future. He receives assurances from the apparitions the witches invoke, such as that he cannot be harmed by any man of woman born and that he is safe until Birnam Wood shall move to Dunsinane. Finally he asks the question that has been plaguing him since the beginning of the play.

Yet my heart
Throbs to know one thing: tell me, if your art
Can tell so much, shall Banquo's issue ever
Reign in this kingdom?

The witches present a show to answer his question. The stage directions call for:

A show of eight Kings, and Banquo last with a glass in his hand. 

Shakespeare wanted to show that there would be many more kings descended from Banquo, but the stage could get too crowded. So the playwright has Banquo holding a mirror--probably just a small hand-mirror--in which Macbeth explains that he sees many more kings stretching back into infinity. The images would be getting smaller and smaller as they receded, but the audience could not see this because they could only imagine it. Macbeth has to describe it, not to himself or to the three witches, but for the benefit of the theater audience.

And yet the eighth appears, who bears a glass
Which shows me many more; and some I see
That twofold balls and treble sceptres carry:

The "twofold balls" allude to the fact that King James I, who claimed descent from Banquo and inherited the crown of England from Elizabeth I, would be ruler of two kingdoms. The "treble sceptres" possibly represent England, Scotland and Ireland, or England, Scotland and Wales.

The "glass which shows me many more" is a simple mirror used to indicate that there are many more kings in Banquo's line of succession than can be conveniently shown on the Elizabethan stage along with Macbeth, the three witches, and Banquo with seven other actors representing his descendants. Only Macbeth can see into the mirror. He explains to the audience that the glass, or magic mirror, shows him "many more" of Banquo's descendants.