For a hydrogen atom, when an electron starts from an excited level $n$ and ultimately reaches the ground state, it can emit photons corresponding to all possible transitions between the available energy levels.

The maximum number of distinct emission lines possible from level $n$ is: $N = \frac{n(n-1)}{2}$ This counts all possible downward transitions among the levels $1,2,3,\dots,n$.

Here, the electron starts from $n=5$. So, $N = \frac{5(5-1)}{2} = \frac{5\cdot 4}{2} = 10$

Therefore, the maximum number of emission lines is: $\boxed{10}$