Based on how much food/water/energy is wasted every day, and the disparate standards of living, I'd say that the possible capacity is greater than 8 billion.
The notion of carrying capacity relies on either the absolute capacity of the environment (i.e. humans are doing everything they can to maximize their population), or the more realistic theory that knows our current systems and infrastructure fall well short of the theoretical carrying capacity.
Technology is/has rapidly been increasing the carrying capacity for humans, who knows what happens if it slows down while population ratchets up.