In a sample, the growth of microbial cells started with an initial concentration of $5 \times 10^4$ cells per millilitre $(\mathrm{mL})$ of the sample. After a certain time period, the cell concentration was found to be $1280 \times 10^4$ cells per $\mathrm{mL}$ of the sample. Assuming binary fission of cells and no cell death, the number of generations of cell growth occurred in this time period is $\text{(answer in integer)}$.
