Is the maximum human population carrying capacity of Earth higher or lower than the current total human population?
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.