Is the maximum human population carrying capacity of Earth higher or lower than the current total human population? Not sure if this is the right stack for this but:
I have been arguing with friends on overpopulation, several are convinced that Earth cannot sustain the current population.  Without getting into all of the ways that we are destroying various ecosystems and polluting, I believe that if we were actively working to optimize the environment and how we use resources, Earth could easily handle many, many more humans.
Such as renewable energy, reducing waste, committing to reversing desertification, and so much more.  But regardless of the behaviors we specifically change, looking at the raw chemistry of the planet, i.e. how much water, oxygen, carbon, plant life, and so on, there is some maximum amount of humans that can live healthy lives.
The FAO estimate that we use 11% of the Earth's surface area for agriculture,  and that that represents approximately a third of the land that 'could' be used (http://www.fao.org/3/y4252e/y4252e06.htm). If we accept that current agricultural production is at a level that allows the current population to live healthy lives that implies that we could triple our population and not starve.
In this very simplistic view I am ignoring the very real questions of inequity in distribution, what constitutes 'healthy' and the potential of changing efficiency in agriculture or transitioning populations away from expensive meat based diets.
However, and it's a really big however, you couldn't make that sort of increase in the area of globe used for agriculture without destroying natural ecosystems, and it's arguable that already much of global agriculture is unsustainable in it's present form, as aquifers are overexploited and soil quality degrades.