The probability of one molecule being confined to the marked volume is
$$
p=\frac{V}{V_{0}}
$$
We can choose this molecule in many $\left(N_{C}\right)$ ways. The probability that $n$ molecules get confined to the marked volume is cearly
$$
N_{C_{n}} p^{n}(1-p)^{N-n}=\frac{N !}{n !(N-n) !} p^{n}(1-p)^{N-n}
$$