In a batch culture experiment, $2$ cells of a microorganism were added to a growth medium. During the experiment, a lag phase of $1$ hour was first experienced by the added cells, in which no cell multiplication occurred. Afterwards, cells started multiplying exponentially with a doubling time of $1$ hour.
After total $8$ hours of experiment, the number of cells present in the growth medium was $\_\_\_\_$ (answer in integer).
