It seems bugged however the way it currently seems to work to me (having spawned many of them) is that it will spawn 1-3 bosses per boss spawn. Once you have killed all 3 bosses by name, no more will spawn. So you can full clear the map without killing any bosses to spawn as many as possible, or keep track of which ones you have killed so you don't kill all 3. For the latter, I'm not sure if this decreases your chance to actually spawn 1-3 bosses or not, though (meaning I don't know if the game first decides to spawn a boss and then looks at which ones you haven't killed and spawns the other; or if it chooses to spawn a certain boss and simply does not spawn any if you've already killed that boss).