Let $M=\{1,2,3, \cdots, 1995\}, A$ be a subset of $M$ and satisfy the condition: when $x \in A$, $15x \notin A$. Then the maximum number of elements in $A$ is $\qquad$ .
6. Let $n(A)$ denote the number of elements in set $A$.

From the problem, at least one of the numbers $k$ and $15k$ (where $k=9,10, \cdots, 133$) does not belong to $A$, so at least $125(=133-9+1)$ numbers do not belong to $A$,
i.e., $n(A) \leqslant 1995-125=1870$.
On the other hand, we can take $A=\{1,2, \cdots, 8\} \cup\{134,135, \cdots, 1995\},$ which satisfies the problem's conditions, and in this case $n(A)=1870$,
$\therefore$ the maximum value of $n(A)$ is 1870.