"Sextillions" Beavis, not "Billions".The chances of an intelligent race with the capability of reaching Earth existing at the same time as ourselves are astronomically slim.
Based on what mathematical computation? There are literally billions of other planets in the universe of which we know nothing.
In terms of the number of solar systems present in the universe, there are something like 300 billion stars in the Milky Way, so if 10 per cent of them have planets there are around 30 billion planets in our galaxy alone, and there are over 100 billion galaxies in the observable Universe for a total of something in the order of 10^21 (that's 1 then 21 zeros) planets in the observable Universe. There is still quite a bit of uncertainty in that number however, and we don't yet know how many of them would look like our solar system.