Bacteria is a unicellular organism which can multiply very fast by binary cell division.
To get a number of divisions required for cell division: you can use the formula: 2

= number of bacteria
To get 256 bacteria:
2

= 256 (you can solve this equation by a calculator)
so:
n = 8 generations
With this formula the first bacteria is taken into consideration since 1 bacteria gives two, then the two give four, then eight, 16...