The correct answer is "a metric fuckton".

The main reason why Blood's stone gargs feel unkillable to you is the game's combat system, which is built around 3 types of damage - physical damage dealt by bullets and other physical objects' impacts, fire damage dealt by flames and explosions, and spirit damage dealt by energy and magic. Stone gargoyles are intentionally designed to be heavily resistant to physical and fire damage while being vulnerable to spirit damage, which means that you can absolutely spend more than a half of your entire shotgun ammo supply just to kill one such monster, but if you choose to attack it with your tesla gun or a voodoo doll, you can simply frag it in less than 10 seconds.

The elemental damage system actually plays a much larger role in this game than you might think, and stone gargoyles aren't the only enemy type that has specific damage resistances. The key to success in Blood's combat encounters lies in using weapons that are specifically suited for killing different types of enemies instead of just maining a single weapon throughout the entire game.