The concept of reincarnation varies across different belief systems and philosophical traditions. In some interpretations, reincarnation is not limited to a single universe or timeline, and past lives can occur in alternative universes or parallel dimensions. However, it's important to note that the nature of reincarnation, if it exists, is largely speculative and not scientifically verifiable.
The idea that there are more people alive now than in the past does not necessarily impact the probability of past lives occurring in our present universe. Proponents of reincarnation often suggest that souls or consciousness can transition from one life to another, regardless of the number of individuals currently living. They argue that souls are not limited by the physical population size of a particular era or universe.
It's also worth noting that the population of Earth has increased over time due to factors such as advancements in healthcare, technology, and improved living conditions. This does not necessarily imply that past lives are distributed exclusively in alternative universes or timelines. The belief in reincarnation encompasses a wide range of perspectives, and it is ultimately a matter of personal belief or religious and philosophical interpretations.