jmc

Each cylindrical pipe with diameter 2 has radius $2/2=1$ and volume $\pi (1^2)(4)=4\pi$.

The cylindrical pipe with diameter 12 has radius $12/2=6$ and volume $\pi (6^2)(4)=144\pi$.

Thus, we need $\frac{144\pi }{4\pi }=\boxed{36}$ pipes with diameter 2 to hold the same amount of water as a pipe with diameter 12.