From the Super Mario Wiki
Initially, three Skeleton Ghosts (five in the PAL Hidden Mansion) are found. After Luigi captures the ghosts, Bogmire transports Luigi to a large circular area to battle him. After Luigi defeats Bogmire, Luigi opens the large chest to find a green club-shaped Key to Area 3.
The Graveyard is has eight tombstones; the top left-most one always contains a 50-HP-restoring Heart for Luigi. In addition, two torches are on either side of Bogmire's tombstones. Fire Elemental Ghosts spawn from these torches.