There are 16 Ghini total in the graveyard. You will find 4 sets of headstones, each containing 3 hidden Ghini and 1 roaming Ghini.

When you touch a headstone which is hiding a Ghini, it will quickly appear. Kill the roaming Ghini after spawning the hidden Ghini to take out all 4 from the set at once.

These spawn locations are fixed and are shown in the map below: