How many times do ghosts appear in Shakespeare's plays? Is it two or five times?

Ghosts:

1. Hamlet (ghost of his father)
2. Macbeth (ghost of Banquo)
3. Julius Caesar (ghost of Caesar)
4. Richard III (Ghosts of Henry VI and Edward IV)
5. Cymbeline (ghosts of Pothumus's father, mother and brothers)

So in the sense of "spirits of murdered people" there are ghosts in Hamlet, Julius Caesar, Macbeth and Richard III.

In the sense of apparitions, whether friendly or malevolent, there are "ghosts" in Cymbeline, Henry VI Parts one and two, Henry VIII and Macbeth.
[editor: NEED REFERENCE FOR HENRY VI and VIII. CANT FIND REFERENCES TO GHOSTS IN THEM ]

Ghosts should not be confused with "ghost characters": people named in a text but who do not actually appear.

[ This is a FAQ, and so this top post has been chosen to update the answer. ]

Amateur, amateur...

Or may be if schoolTEACHERS can be called scholars?
[..the whiling schoolTEACHER with his satchel
And shining morning face
Creeping like snail
Unwillingly to school.]

Well, that may be as YOU like it, Flem-ish, but Old Will must be turning over in his grave. :)

He is a spirit of the air. In I.ii.250-93, we learn that Ariel was once the servant of Sycorax, a wicked sorceress who had imprisoned the spirit in a "cloven pine" for refusing to fulfill her "earthy and abhorr'd commands" (I.ii.277,273). Ariel remained trapped inside the tree for twelve years, crying out in pain, until Prospero arrived on the island, released him, and bound the airy spirit to his service. Thus at Prospero's command, Ariel stirs up the tempest which strands Alonso and his followers on the island (I.i). Again acting on his master's instructions, he beguiles Alonso's son,...

Ariel certainly wouldn't fit the classic view of a "ghost", but rather some type of a natural spirit.

Stage directions for the following plays:
(Act,Scene,Line)

Richard the Third....... (11)

V,3,3592 [Enter the Ghost of Prince Edward, son to King Henry VI]

V,3,3601 [Enter the Ghost of King Henry VI]

V,3,3611 [Enter the Ghost of CLARENCE]

V,3,3622 [Enter the Ghosts of RIVERS, GRAY, and VAUGHAN]

V,3,3634 [Enter the Ghost of HASTINGS]

V,3,3642 [Enter the Ghosts of the two young Princes]

V,3,3653 [Enter the Ghost of LADY ANNE]

V,3,3664 [Enter the Ghost of BUCKINGHAM]

Hamlet(1)

I,1,51 Enter Ghost.

Macbeth (1)

III,4,1321 [The GHOST OF BANQUO enters, and sits in]
MACBETH's place]

Julius Caesar (1)

III,3,2304 [Enter the Ghost of CAESAR]

This is what I could glean without reviewing ALL his text!

http://www.opensourceshakespeare.org/views/plays/plays.php

There are at least five Shakespeare plays with ghosts:
Hamlet (ghost of his father)
Macbeth (ghost of Banquo)
Julius Caesar (ghost of Caesar)
Richard III (Ghosts of Henry VI and Edward IV)
Cymbeline (ghosts of Pothumus's father, mother and brothers)

Source: Shakespeare Complete Works; RSC/Macmillan

Flem-ish above posted that HENRY VI and VIII have ghosts, but I can't find any to verify this answer. Anyone able to prove there are ghosts in these?