If the average number of past lives for humans currently incarnated is 423, what is the average number of future lives and the average number of future extensions of the parallel lives, for each person?

There is an equal number of future life extensions for all past lives. So each past lifetime has a new extension running. There is a future extension of the current life, and then future extensions of future lives. The future lives are much smaller numbers, and the reason is not only for the eventuality of Ascension that will change the nature of the aggregate parallel lives by moving into another dimension, but the potential limitation on the future of the current iteration of humanity posed by the encroachment of the interlopers to drag down human progress and lead to their destruction. It is changing future potentials dramatically. This is constraining the possible future lives and their future extensions as well. So we would put the total figure for the equal number of future lives and future extensions of future lives, to be at about 25% of the total of past lifetimes.