I see a couple more assumptions here as well as the stated one, that may or may not be relevant but I don’t hold them as necessarily true all the time, so they’re going to affect my answer.
a) Every human has one soul (or maybe, few enough souls that the total number is increasing over time) and
b) the only souls in existence are all on earth.
c) The only souls on Earth are only in humans
Definitions I’m working with: A spirit is sentient energy, A soul is a spirit fused with animal life (physical vessel), emotional, mental and other constructs that by themselves are not sentient but can be used by the spirit to manifest an incarnate reality experience. A human is a physical human body with a soul. A cat is a physical cat with a part of the soul of the Deva of cats. A Sirian is a Sirian body not on Earth with a soul. A Fae is a being from another dimensional plane of Earth with a soul etc… and I’m an animist so I do see spirit inhabiting things that most see as soulless, which is to me a modern interpretation of the Fae. Like, my car, incorporated or social organisations, and software products.
So I think there are two things. 1) there are more spirits in existence than 8 billion anyway, and 2) spirits are being created all the time. They can be recycled back to source energy also. And, they can leave and enter this universe changing it’s overall spirit population as well. E.g. the ‘dead’ daemons like Apep. So, for this:
In that case, are you suggesting that there are a finite number of spirits that swap between planets, i.e. for our population to go up another on another planet must go down?
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That said, presumably all of these spirits came into existence at some point.
No, I think the total population of spirits in this particular universe is variable over it’s timeline, in a very dynamic sense, via multiple methods of creation, migration and conversion to other forms of energy.
Do you believe there is a god or set of gods who are responsible for creation of life? I have a hard time believing that because it’s been shown that inorganic chemicals, when mixed in ratios similar to those of primordial Earth, can self-assemble to engage in biological reactions.
Life and spirit are two different things. The former provides a vehicle for the latter in physical reality. Not all life needs to be directly spirited, but since the point of it it to provide spirit with a vehicle, it quickly gets sick and dies without it.
I don’t discount the “Eternal source” idea, but it’s more of a, ‘nothing “created” all this because it was always here’ - with no beginning and no end. I do think it’s outside the multiverses. Energy cannot be created or destroyed, only converted from one form to another. Science with it’s very limited instruments has figured that much out - my first degree was in Theoretical Physics specifically because I’ve always been a seeker and wanted to know how the universe worked, but you hit a wall with it quite quickly, as you do with many lines of inquiry boxed into a given framework. I don’t think new souls in this universe are directly created by source, neither are they unmade, but are rather generated and unamde by the activities of the sovereign daemons in it including those incarnate as humans.
Yet I believe in beings I cannot see. Make sense eh? 
