There are an unlimited amount of souls. A soul is spiritual by nature. It is part and parcel of the spiritual Whole, God. The spirit-soul is the lifeforce of matter. In other words matter is useless without the intervention of the spirit-soul. For example, a car cannot act without the aid of a person (spirit-soul), it remains just a lump. A body without a soul is useless, it is a dead body. The symptom of the spirit-soul is consciousness.
God is pure spirit, the Supreme (male) it is He who injects life into the material energy, (female). From this point on, the universe(s) becomes abundant with life. We are no-different from the Supreme qualatively. Where we differ, is that God is infinate and we are finate. An example is if we take just a drop water from the atlantic ocean, it would be identical to the rest of the ocean, qualatively, but would not be as great as the ocean quantatively.
So our position is that, as spirit-souls we are separated parts and parcels of the the Supreme Soul (Brahman).
Why are we separated?
Because we decided we wanted to enjoy, like God. In other words we envy God, so we became rebelious, like Lucifer, Ravanna or Kamsa. So God created a universe(s) so we can act out our fantasies. In order for us to live in the material atmosphere, we require bodies, so the bodies are given to us by (mother) nature. The seed is provided by God.
As soon as there is sexual intercourse a designated soul is waiting to enter into the womb of the mother. I say designated, because that person (spirit-soul) has the right qualifications to be born unto those parents, be they ants, animals, humans or demigods etc. His parents are chosen, either according to his actions (if human) in his previous life (karma), or if he was a plant, animal, insect etc...he then moves up the evolutionary scale until he obtains a human body. It is stated in vedic literature that there are 8,400,000 species of life.
The spirit-soul never changes, because he is part and parcel of the Supreme Soul he is eternal, never comes into being or never destroyed. Matter on the other hand, has to be created and ultimately has to be anihilated, this body is made up of all the variegated material elements eg; air, fire , water etc...
After the dissolution of this material body, the soul has to accept another body according to his actions.