Earth’s population is closing in on 8 billion people. Do you think Earth could sustain adding more people (say an additional four billion)? Is there a number in your head where Earth would be facing a grave crisis? What solutions could be there?

Earth’s population is closing in on 8 billion people.  Do you think Earth could sustain adding more people (say an additional four billion)?  Is there a number in your head where Earth would be facing a grave crisis?  What solutions could be there?