Reply #1: the explanation that I have heard

[momzno1]

is that we are not the only planet, or "school of learning" in the Universe, so the number of souls available for incarnation is huge.
Some believe that we have chosen to reincarnate en masse in this time period because of all of the "learning opportunities" and changes in the world at this time.

brian weiss wrote a bunch of books on past life regression that are great. Many Lives, Many Masters -- is his first book. He was a traditional psychiatrist who came to believe in reincarnation after he accidentally regressed a patient to a previous life when he suggested she go to "the time the problem started" while under hypnosis.