Antoni van Leeuwenhoek, the great Dutch scholar, provided the first serious estimate on April 25, 1679, in Delft: 13.4 billion. Unfortunately—and most interestingly—the sequence of subsequent estimates does not converge to a well-defined number.Here's a graphic with the different numbers people have come up with over time.
Its interesting that the mean is still near van Leeuwenhoek's number. My personal calculations, when working on a paper critical of the ecological footprint, suggests that using a footprint standard where carbon is sequestered by land, the Earth could easily sustain all 6.6 billion people, each consuming like Americans. Depending on how high crop yields can get (they are increasing at an incredible 2%+ per year for the last 40 years in Europe and the US), the number could be even higher.
Keep in mind this calculation is for every person consuming like Americans. If we just have an increase of the average world person, the Earth could sustain over 13.2 billion people, which is eerily close to van Leeuwenhoek. If we drop the idea that carbon can be sequestered, this number jumps considerably higher.