6. Let $M=\{1,2,3, \cdots, 1995\}, A$ be a subset of $M$ and satisfy the condition: when $x \in$ $A$, $15 x \notin A$, then the maximum number of elements in $A$ is $\qquad$ .
1870
6.【Analysis and Solution】Let $n(A)$ denote the number of elements in set $A$.
According to 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 conditions of the problem. In this case, $n(A)=1870, A$ satisfies the conditions of the problem, and at this time $n(A)=1870$.
Therefore, the maximum value of $n(A)$ is 1870.