How Many Souls Are There?

Let's assume that our existance is in the form of a soul - our bodies are not immortal in the physical sense, thus after we die, we should exist in a different form. We do not fully know or understand what is not seen, so there are many beliefs and theories, all unproven. Some believe we will be a spirit after death. But what is the difference, if any, between a soul and a spirit? Since we do not know, let's just say that we will continue as a soul after death. If we do not believe in existance after physical death, then life would be meaningless on this earth. So let us assume that there is life after physical death.

We know that the population on the earth had been increasing over the years. Does that mean that every birth is a creation of a new soul somewhere unknown to us? If we add up all the figures of the world population for the last 1 million years and the figure comes to 100 billion, does that mean that there are at least 100 billion souls in existance (not counting others in the universe, unknown to us)?

Some possibilities are:

a. a soul can exist in more than one physical person in this universe; that would mean less souls;
b. a soul can come back to earth after death (reincarnation); that would also mean less souls.

Of course, we do not know the truth. But assuming that there are at least 50 billion souls in existance, then I wonder where these souls are, what they are doing, and what is the purpose (if any) of existance? How many news souls are created, and at what rate?

Some teachings that claim to be Buddhist in origin (I am not sure if Buddha did give them) claim that there are at least 7 levels of spiritual existance, with the physical level (our level) as one of them. The lower levels (using our earth as reference) are hell, while the higher levels are heavens. The purpose of our lives here is to improve our spiritual level so that we can reach the highest heaven, where, it is claimed, enlightened souls like Buddha, Jesus, etc, are living. These are souls who have the choice of coming to earth as a human being to bring teachings that will help us grow to be better spiritual beings (I find it strange that after Jesusand Buddha, there had been not other enlightened souls on earth; those who claim to know spiritual truths often quote only these two persons; this again leads me to believe that they were more like robotic beings sent to earth by some advanced race in the past; that also explains why they could fast for long periods, be crucified and suffer all kinds of punishments, and appear to be superhuman. I hope mankind will create such a robot in the near future, and then our views on these people will change).

So again I wonder, how many souls are there, and are new one created all the time?