What does that sign mean?

If we let $a$ and $b$ be digits of a number, $ab$ # $=ab+ba$ . So $987654321$ # $=987654321+123456789=1111111110$

Given digits $a_1,\ldots,a_n$ , $\overline{a_1\cdots a_n}\#=\overline{b_1\cdots b_n}$ , where $b_k=a_k+a_{n+1-k}$ .

Therefore, $987654321\#=101010101010101010$ , so the answer is $\boxed{9}$ .