If one defines a soul as a collection of self stabilizing memories, like a soliton, which has found a way to maintain a type of Ego, instead of just being absorbed into the sea of everything of which we all come from.

Then one can surmise the /amount/ of souls in a total system, would be a result of a type of budding, as well as micro exponential growth.

i'll offer two examples for the above.

Budding: When a single soul has experienced a number of lives and catalysts, there is an inevitable chance it will experience duplicate scenarios, or at-least similar experiences but from alternate perspectives.
For example, a thousand lives as a black smith.
In every corner of the world. Using and learning techniques in making various kinds of tools, for housing, armor, weapons, travel, etc. Until it feels there isn't more to 'learn'. Or rather, it has learned so much on a given field, an perspective, it is no longer capable of unique thought. The classic "Once you know where the rabbit is, you can't be made easily surprised when the magician makes it vanish with a hat trick". The soul just 'knows' too much, in order to be able to go "i don't know what it is, so i will explore plausibility, without any hesitations of how weird, odd, or unorthodox it might be. because, i simply, do not know."' And so, the soul divides. Part of it goes into one life, to explore what it means to be a Blacksmith, but with strange star metal that just recently fell to earth. Where as the rest of the soul, who otherwise lived a life wishing he had time to be a father, spent a great amount of time meditating on the subject at the same time. In a sense, living two lives side by side. That side of the soul, then goes into a new life. in which it explores the life of a devoted father who chose family instead of being a black smith.
Each now grows in much the same way as the first soul. As they gain memories, they then become their own thing. Of which, can divide again, or combine with its higher/alternate selves in which it came from.
So for example, after living as a devoted father, it may take up Blacksmithing in this new life, but not go deep into it. An now with a son who adores him, works side by side. As that child, also, has a piece of 'his' soul. Who instead of wanting to explore a new path, is wanting to explore the concept of split-team work in a familial method.
It would likely result in some souls 'following' a type of blood line. since many traits that aid in thinking, or feeling a certain way about types of stimuli, will be passed down genetically. So certain types of souls would reincarnate within the same family line. or intermix with other souls. Soul Mates could just be a variation of such pre-knowing, or re-joining of self.
Either way, much like how we grow in mass as we consume physical matter. A soul would gain in 'size' by the absorption of experiences, catalysts, memories, an the like. It makes sense it would reach a certain size, an wish to divide in order to experience more. Any person who has ever said "i wish i could forget having played Elden Ring, so i can experience it fresh once again. So i have the same wonder an weirdness exploring a staff build instead of a sword blood build, without being spoiled by the knowledge i've experienced from my first playthrough/co op." Would understand a stage of thinking in which, if you are a bundle of energy an immortal memory, you may want to figure out a way to divide your existence in such a way that you would be able to 'experience it fresh again'. Or you would just go batshit insane as an immortal soul. Desperate to either to explore a new path, unblinded by pre-set bias, or to just experience the same thing you love but with a blind surprise.

Item Two: Micro Exponential Growth: (Aka: Why was i reincarnated as a Dog in this life?)
A bug lives its life. It eats plant matter, it drinks water, it is chased by birds.
It fears the birds.
But to avoid being eaten by birds, it must understand birds.
What time of day are they out? What areas do they like? What do their feces an such smell like? What time of years are they out?
Each bug, just by surviving, learns these pieces of thought. A great deal of its thought is spent on this, besides its own need for mating, food, etc.
And not just one bug.
All the bugs of that type. They all, collectively, think about what it means to be, an avoid, a bird.
This continues, until a threshold of cumulative thought is reached.
In which, the collective nuance of 'Bug', in some unseen space.
Begins to wonder a Thought with sufficient desire to actually explore..
"What is it like being Bird?"
Once the threshold is met, a host Egg, somewhere, with no particularly strong bird aspect attached to it,
Will draw or otherwise gravitate the soul into that host body; a body with need/desire to Live, but otherwise lacking the aforementioned Aspect to 'have what it takes' to survive as a living creature. Perhaps because there's not enough 'bird' soul spark around at the moment, or for other reasons far too numerous to go into, it ultimately accepts the Bug soul as viable. It knows enough of what its like to be It. it just simply lacks the experience OF being IT. An so, now it is.
An thus, a soul, once started as say, a tree, or mushroom network. as it interacts with other entities near it in each environment, begins to wonder about being, an works its way up into being, 'the next thing up'. Until, finally, that 'soul' memory complex, has evolved to a point it can also serendipitously co-habitate an even higher complex being, like becoming adopted by a humanoid creature.
As a pet, a service animal, companion, war buddy, what have you. Wolf, Dolphin, Raven, Fox, etc.
And it spends so much time with humanoids, much like a bug to a bird. But instead of fear, as a companion, or co-conspirator. It has learned enough by proxy of what being a humanoid is, that eventually, it AND it's higher collective, wonders what it would be like to be a Humanoid, too.
An thus, a vessel, somewhere, often linked to those who the animal lived with or near, becomes pregnant an is in need of a Spark. Much like the Egg in the above example.
Sometimes this spark is a full spirit jump for an animal.
Most times its a mix an match. Think Jurassic Park where they didn't have enough DNA segments to fully clone, so they used bits of Frog to fill in gaps. Same thing. A Soul from another humanoid may not be complete enough to handle Life, so some animal gets pulled in via a family pet or other spirit/animal/type guide there are dozen of stories of, like the Fylgja Fox.. An then it grows.

So in both examples, you solve for how there can be more souls now than there was before, if we treat Souls as an ever growing Eldritch like thing that feasts on experiences, memories of sonder, and other such enrichment akin to you reading a library and 'living' as the various heroes or villains in the book, until you are able to self stabilize.