We can all agree the population can’t continue to grow. We can also agree it probably needs to shrink, especially by the time this starts making a difference.
However, if it shrinks too rapidly, there’s a lot of potential disruption of society and economy. If it continues to shrink, it could be a serious problem for all of humanity.
We should make changes now to encourage more people to have kids. The goal should be a slow, controlled decrease, to level off, without major disruption
Personally, I like 6B as a good place to plateau. We’re probably already beyond the planet’s carrying capacity so need to be less than today. However a lot of the advancements in society (technology, space, medicine, science, innovation) really require a fairly large population. Establishing a number ought to be someone’s thesis, but in the meantime: 6B