Answer: 25

\begin{eqnarray} 8 \; \text{stadia} &=& 8 \times 125 \; \text{passus} \\ &=& 8 \times 125 \times 5 \; \text{pedes}\\ &=& 5000 \; \text{pedes} \end{eqnarray}
Marcus has to walk $5000 \; \text{pedes}$.

$1$ circuit of his atrium is $50 \times 4 = 200 \; \text{pedes}$ long, so he has to walk round his atrium $\frac{5000}{200}=25$ times.

In Roman numerals, $25$ is XXV.