Many parts of the world are very sparsely populated. The north American continent alone would easily double or triple its current population. We'd have to be smart about it, think carefully where we put new cities etc so that we hedge against climate change... it would be easily doable.
Africa too, is capable of supporting a much higher population IF we want to get really serious about doing so.
Africa too, is capable of supporting a much higher population IF we want to get really serious about doing so.