The Supernatural Universe would give you the impression that there is a limited amount of anything associated with the spiritual or supernatural realm of existence. That is a misnomer perpetuated by the very nature of the medium we use to watch the show. If the show ran for another twenty years, we would have no shortage of monsters, angels, or demons for Hunters to battle valiantly against.
Here's why:
Angels
Whenever we see Angels, for example, they all appear to know each other and so when they die or are destroyed, they all appear bereaved (unless they are less than sane).
What we fail to remember is this: there are countless Angels populating the heavens. Their apparent familiarity is due to their limited omniscience of each other.
All Angels are aware of every other Angel, and this is key, when they put their mind to the task.
So to our human minds, they appear to be a close and local family. But their family is far larger than we can comprehend and they have had millions of years to be acquainted and greater than human levels of comprehension to instantly interact.
And even with all of that familiarity, they can still be surprised by the choices that other Angels make.
With that said, there are likely to be millions of other Angels we haven't met and have chosen instead to keep their heads down during these events and insurrections caused by Micheal's, Gabriel's and Lucifer's struggles for power and dominance. There shouldn't be a shortage any time soon.
Demons
Since the original pedigree of demons implies they were spawned originally by Lucifer, who was immediately locked up for transforming Lilith, there must be a method by which new demons are created that does not require the direct interaction of Lucifer.
The show implies a demon was once a human soul that is tortured until their humanity is lost and they totally embrace being evil.
There does not seem to be a lack of evil humans on Earth willing to trade their souls for power, make deals with demons or are so unrepentantly evil that Hell cannot make further use of them, our Demon supply should be not only constant but growing.
Fortunately for us, Demons don't like or trust each other, so they are just as inclined to kill each other as us, a built-in population-limiting factor.
Monsters
Yes, EVE was called the Mother of Monsters. But given what was hiding in Purgatory during the Leviathan Saga, there are likely plenty of other monsters and creators of monsters we have not met yet.
They are likely hiding on other continents, in other religions, in other mythologies feasting upon the tender and delicious sweetbreads, spiritual effluvia, blood, semen, psychic energies, petty fears and anything else that makes Humans such a tasty monster chow.
I am pretty certain they breed faster than Hunters can kill them. Most seem to come with their own built in propagation systems (vampires create new vampires by biting them, werewolves create new ones with a scratch or bite) so they don't need a Monster Creator like EVE to keep producing new ones.
Ghosts
A ghost or spirit remains connected to the mortal realm because it is attached to it through unfinished business or an unwillingness to move on. They may, as Dean nearly did and Bobby did do, refuse to accompany the Reaper who is tasked to help them move on. Many have a desire for revenge against those who they perceive to have caused their deaths, or will target people like them. Others like Dr Ellicott, Dr. H.H. Holmes, and Nurse Glockner were evil in life and continued this after death.
- Ghosts can be harmless or malevolent. Given the number of ghosts the Winchesters have had to dispatch, it is the rare ghost that is NOT trying to kill them. Since ghosts are human souls that have not moved on, there should be no lack of them.
- Demons are not "super-charged" ghosts. The method of destroying a spirit works on Demons whose physical remains can be found because the demon is the spirit of a human who has been corrupted and has lost all sense of its humanity. If you can find their bones, sanctifying/purifying those bones will work just like on any other spirit.
- But demons are much higher on the spiritual food chain and far more dangerous. Powers that ghosts may manifest with a bit of practice are part and parcel of the demonic package. This includes possession, telekinesis, superhuman strength during possession, awareness of other supernatural beings.
The beauty of Supernatural is they have created a show where they can always plumb another religion or society's supernatural realm for entirely new breeds of monsters unfamiliar to their viewing public. I give them good marks for the range of creatures and their willingness to experiment. When the show ends, it will be missed.
