In a significant 2004 paper reviewing 69 studies on world human population and carrying capacity, the authors estimated that the sustainable carrying capacity of humans for Earth is about billion.

About 7.7 billion.

Explanation:

The carrying capacity is the biological specie is the environment that can support the maximum diversity of the species and is the capacity that the environment can sustain indefinitely. As per the united nations report the carrying capacity of the environment of the humans is about 10 billion and is measured by the ecological footprint. And would take about 1.5 earth to sustain depending on the present levels of the consumption.

D.