This depends on how you define the word 'ghost'.  If this is interpreted as the spirit of a person who has died returning to see the living, there are five plays that contain ghosts, giving a rough total of 18 ghosts themselves.  These plays are:

Richard III: the ghosts of Prince Edward, Henry VI, Clarence, Rivers, Grey, Vaughan, Hastings, the two young princes, Lady Anne and Buckingham appear to Richard and Richmond when they are asleep.
Macbeth: Banquo is in Maceth's seat at the banquet
Hamlet: his murdered father appears on the castle battlements and again in the Queen's bedroom
Julius Caesar: Caesar appears to Brutus
Cymbeline: the ghosts of Posthumus' parents and two brothers appear to him in prison.

However, in addition to these, spirits appear in a number of other plays - for example there are the apparitions that the witches conjure up in Macbeth and the spirits that appear to Jeanne D'Arc before Angiers in Henry VI, Part 1.  In some cases ghosts/spirits do not take the form of a physical presence on the stage, but still haunt the living - for example Tybalt in Romeo and Juliet.