All it takes is one bacteria. The cell's divide by binary fission where the microbes DNA splits to form a new cell.
They have 1, because they are Prokaryote. http://en.wikipedia.org/wiki/Prokaryotic
It depends on their rate of growth which will depend on the matrix and temperature as well as the species - none of which are specified.
theres none except about 1000-2000 bacteria in the penis and vagina thats y u get hiv andd aids