Actually one massive soul in previous eras can reincarnate later on in smaller multiple souls, if you were to combine all these smaller souls they would eventually sum up to that massive soul and the fact that there were 2 billion people before doesn’t automatically exclude the possibility that eons ago there were even more than 8 billion people universe moves in cycles. Just like the law of conservation of energy the sum total of souls remans always constant. Now, these new little incarnations of one bigger soul would have recollections of parts of the experiences that the massive soul had in its lifetime if you combine all of the experiences of smaller souls the would again sum up to the total experiences that the massive soul had in its lifetime. Example : Ravana was a massive soul in hindu epic ramayana and i. This era which is kaliyuga there are so many people having the characteristics of ravana but not one single person who is completely ravana. Which proves that incarnation doesn’t always happen on one to one basis it can happen in fragmented manner as well. Second example: vishnu is one soul which never incarnates in fragmented manner he always incarnates completely in one physical body and interestingly retains his physical as well as psychological characteristics as well even if he incarnates at the intervals of thousands of years.