Initial: $n_1 = 2$, current $I_1$, radius $r_1$, $B_1 = 8$ T.
Final: $n_2 = 4$, current $I_2 = 2I_1$.

Since total wire length is same: $n_1 \cdot 2\pi r_1 = n_2 \cdot 2\pi r_2$
$$r_2 = \frac{n_1}{n_2} r_1 = \frac{r_1}{2}$$

Magnetic field at centre: $B = \frac{\mu_0 nI}{2r}$
$$\frac{B_2}{B_1} = \frac{n_2}{n_1} \cdot \frac{I_2}{I_1} \cdot \frac{r_1}{r_2} = \frac{4}{2} \times 2 \times 2 = 8$$
$$B_2 = 8 \times 8 = 64 \text{ T}$$