The physicists, Andrei Linde and Vitaly Vanchurin, calculated the number by first going all the way back to the Big Bang. Linde and Vanchurin posit that the stellar organization and physics of our universe resulted from small perturbations in the otherwise uniform mass of matter and energy that existed milliseconds after the Big Bang. So, the number of possible variations of those perturbations represents the upper limit of possible alternate universes, or about 10^10^10^7 possible alternate universes.