A PCR tube starts with 8 molecules of the template…

Step 1. (a) $8 \times 2^{12} = 8 \times 4096 = 32768$ molecules.

Step 2. (b) We need $8 \times 2^n \geq 1\,000\,000$. For $n = 16$: $8 \times 65536 = 524288$, which is not enough. For $n = 17$: $8 \times 131072 = 1048576$, which is enough.