Question 232, Set $P=\left\{x \mid x=7^{3}+a \times 7^{2}+b \times 7+c, a 、 b 、 c\right.$ are positive integers not exceeding 6 $\}$, if $x_{1} 、 x_{2} 、 \ldots 、 x_{n}$ are $n$ elements in set $P$ that form an arithmetic sequence, find the maximum value of $n$.