The following information pertains to an upcoming version (NetHack 3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.High priests are now referred to as "high clerics".
There are exactly four in the game: three corresponding to each of the deities of the player's pantheon, who appear in the High Temples on the Astral Plane, and the high priest of Moloch that appears in Moloch's Sanctum. In addition to those four, aligned priests can grow up into high priests.
The high priests on the Astral Plane are always generated peaceful, and will remain that way unless you provoke them. The high priest of Moloch is likewise generated peaceful, but will become hostile upon your entering the temple.
The high priest of Moloch holds the Amulet of Yendor, and it is extremely likely that killing them will be necessary to obtain the Amulet. This can be difficult, and players will tend to save a wand of death charge for the specific purpose of killing them; this will not work if the priest is generated with a cloak of magic resistance.
As mentioned above, the high priest of Moloch is generated peaceful, but turns hostile once you step into the room holding the high altar of Moloch, so be prepared before opening the door. This also means that killing them before stepping in the temple counts as murder for lawful and neutral characters.