12. If $p$ is the smallest positive prime such that for some integer $n, p$ divides $n^{2}+5 n+23$, then $p=$
13. Let $M=\left\{a_{1}, a_{2}, \cdots, a_{m}\right\}, a_{1}>a_{2}>\cdots>a_{m}$. If $m \geqslant 8$, since multiplying each number by -1 does not change the property of $M$, we can assume without loss of generality that $a_{4}>0$. Then for the set of three numbers $a_{1}, a_{2}, a_{3}$, it is clear that only $a_{2}+a_{3}=a_{1}$ holds; and for the set of three numbers $a_{1}, a_{2}, a_{4}$, it is also clear that only $a_{2}+a_{4}=a_{1}$ holds, thus $a_{3}=a_{4}$, which is a contradiction! Therefore, $m \leqslant 7$.
On the other hand, the set $M=\{-3,-2,-1,0,1,2,3\}$ satisfies the condition. In conclusion, $M$ can have at most 7 numbers.