Except for Lady Macbeth's sleepwalking scene, Act V of Macbeth is a sequence of scenes that, at first, cross-cut between Malcolm and Macduff and their army of English soldiers and Macbeth in his castle preparing for battle, and then culminate in the battle for the throne of Scotland between the forces of Macbeth and the forces of Malcolm and Macduff.
There are a few reasons that one could say that Macbeth does not fear the approaching armies and their war drums. First, it isn't because he has more soldiers. In Act V, scene iii, a servant enters, one who must look petrified. Macbeth immediately calls him out.
The devil damn thee black, thou cream-faced loon!
Where got'st thou that goose look?
There is ten thousand--
And though Macbeth is apparently greatly outnumbered, he is not disturbed or fearful. He believes the prophesy -- that he will not be defeated until the Birnam wood comes to Dunisnane. He knows that trees cannot pick up and walk, but he doesn't consider that the trees will be cut to use as camouflage. So, mainly, he is not disturbed because he has the prophesy on his side.
He is also not disturbed because he is a soldier. He says to Seyton in this same scene iii, "I'll fight till from my bones my flesh be hack'd." Macbeth has been trained to prepare for battle with a mindset to win, and this is what he does in this case.
And finally, he is not disturbed because he has basically lost all access to his human emotions. His very famous speech in Act V, scene v (the "Tomorrow and tomorrow and tomorrow" speech) that reveals him to be a man who sees life as a "walking shadow," "signifying nothing." So, he is not capable, at this stage of the play, of being disturbed by anything -- not the ensuing battle and not even his own wife's death.