First question: What should be the sustainable level of human population on Earth?
That depends totally on the socioeconomic systems by which people live and on the consciousness of the people. As an example, people living in a third world country through subsistence farming or otherwise struggling to survive have a very small ecological "footprint." If their way of life is locally sustainable (i.e., not depleting the local resources) and they're conscious of and committed to not exceeding the carrying capacity of those resources, their population would only be limited by the availability of local resources and their ability to use them to greatest advantage.
But let's say a family from Mexico or another Central American country is able to migrate to the US and views the American way of life - with its large houses and SUVs - as a desirable way of life worth striving for, their presence on the earth almost immediately becomes non-sustainable. At least this is true once they're able to afford this over-consuming way of life.
So we have two modes of living - of acting to meet our needs - that stand in stark contrast to each other. One I call the "shopping mentality;" the other could be called living consciously. Shopping mentality is looking outside one's self for what one needs and is not based on a true awareness of what our true needs are or what provides true satisfaction, contentment, fulfillment, etc. It is living by looking at the existing systems and seeing how we can fit into or take advantage of those systems to get what they have to offer. It is also coming from fear, scarcity and powerlessness, viewing the products of those systems as a way of distancing ourselves from what we fear: poverty and helplessness.
Conscious living starts with our real needs, includes our real feelings, and incorporates an inclusive awareness of our fellow humans (or community, if such exists) and the natural world. We are aware of ourselves as individuals and as integral parts of our physical and social contexts or environments. It is a way of life that becomes much more feasible within a community of mutual support where we're able to create with others a way of living that fits who we are and truly meets our needs. This is quite possible, as we all have essentially the same real needs manifesting in many different ways.
Question 2: On what grounds would you justify your answer?
This is what I've come to as a result of many years seeking to understand why we and the society are as we are, what kind of human society we could create and how to get from here to there. Certainly the earth has a finite carrying capacity, but we cannot make any real assessment of that capacity based on the present socioeconomic system that exists much like a cancerous malignancy devouring its host. So we can only imagine what life on earth might be like if human society were designed and built in a sane way. The picture becomes vastly different if we're seeking efficient, harmonious ways to meet real human needs rather than working to sustain a monstrosity that exists primarily for the benefit of a rich and powerful ruling class.
CSummer