Answer
$2$
Work Step by Step
As we see in the figure below, the focal length is twice the radius of the sphere. Hence, $s'=2R$
And since the paraxial light rays are parallel, $s=\infty$.
Recalling that
$$\dfrac{n_1}{s}+\dfrac{n_2}{s'}=\dfrac{n_2-n_1}{R}$$
where $n_1=n_{air}=1$, and $n_2=n_{sphere}$
Plugging the known,
$$\dfrac{1}{\infty}+\dfrac{n_{sphere}}{2R}=\dfrac{n_{sphere}-1}{R}$$
$$0+\dfrac{n_{sphere}}{2 \color{red}{\bf\not} R}=\dfrac{n_{sphere}-1}{ \color{red}{\bf\not} R}$$
$$n_{sphere}=2n_{sphere}-2$$
$$n_{sphere}=\color{red}{\bf 2}$$
