Robert Heinlein, the science fiction writer, opined that the planet could support a QUADRILLION humans if it were efficiently managed, which is about a million times as much as the current population. Much of this efficiency would Obviously, that scenario would never play out.
But I myself have proven that you can put the entire population of Earth into the state of Texas, with a population density the same as the average American suburban home (2,000 sq. foot home on 4,000 sq. foot lot @ 4 people per home). The calculation did not include areas of water like lakes or eddies.