\[ \begin{align} \sigma&\ge\frac{P\times S}{n\times\frac{\pi}{4}d^2}\\ &=\frac{8000\times4}{n\times\frac{\pi}{4}\times\left(12\times10^{-3}\right)^2}\\ &=20\ \rm{MPa}\hspace{30px}より\\ \end{align} \] \[ n\ge14.147\\ \] \[ \therefore n=15\ 本 \]