I am pretty sure Buddhism doesn't say that new souls can not be created. Also I am pretty sure that not every Buddhist sect believes in reincarnation, probably because of issues like that.
In Buddhism, linear time is illogical, except as needed conventionally to exist in this world as we perceive it, therefore one can take rebirth at any point in time, and if the Earth is beginning to fail to support human life, is coming to such an end, then some take rebirth on other worlds.
You might find the Sanghata Sutra interesting in letting you learn some of what we believe. There are also simpler books that teach the concept of "rebirth" as we understand it.
It's a reconstitution of aggregates. In Buddhism, birth and death are explained in terms of the five aggregates of for rm, feeling, perception, consciousness, and mental formations. When those five aggregates come together, that is birth. When they dissipate, it is death. So there's no soul as such going from life to life. It is just aggregates congregating for a while, dissipating, and reconstituting in some other fashion.
In Buddhist cosmology, there are more sentient beings than just humans. There are gods, demo-gods, titans, animals, ghosts, and denizens of hell. Any one of those sentient beings can be reborn in any other realm depending on their Kama.
So there are two things at work here: impersonal process of recombining aggregates, and Kama, which is intentional action.
There is no place where the soul starts and begins, souls can fragment, parts of your soul can be living one life and other parts another, also many come back from the spiritual realms as the opportunity presents, and many highly evolved animals are getting a chance to be human...
My teacher, a Tibetan Buddhist monk since age 12 in the Dalai Lama's monastery, shrugs his shoulders, smiles, and says simply, "I don't know."
Sometimes he adds that it doesn't really matter ... what matters is what's here and now and what we're going to do with it. But he's always quite amused when any of the students ask speculative questions that are not related to Buddhism (which is only about finding freedom from suffering).
According to accumulated Kamma, spirits may be reborn in heaven, hell, animal realm, human realm, Deva realm or ghost realm. Spirits (Consciousness) rebirth is not confined only to heaven or hell. The law of Kamma is applied to every being alike. Lord Buddha once mentioned that hell was the mostly populated realm, out of the all above mentioned ones .Forget about number of being in all the realms but, just think of number of ants and bugs living in this world and the number of human beings. Which is the highest? I hope this would help to solve your question.