6. Given that the three sides of $\triangle A B C$ are $8, 12, 18$, and that $\triangle A_{1} B_{1} C_{1}$ also has one side of length 12, and is similar but not congruent to $\triangle A B C$. Then the number of such $\triangle A_{1} B_{1} C_{1}$ is ( ).
(A) 0
(B) 1
(C) 2
(D) 3
6.C.

Let $A B=8, B C=12, A C=18$. By the similarity of two triangles that are not congruent, we can set
$$
\frac{A B}{A_{1} B_{1}}=\frac{B C}{B_{1} C_{1}}=\frac{A C}{A_{1} C_{1}}=q \neq 1,
$$

i.e., $\frac{8}{A_{1} B_{1}}=\frac{12}{B_{1} C_{1}}=\frac{18}{A_{1} C_{1}}=q \neq 1$.
Since $B_{1} C_{1} \neq 12$, it can only be that $A_{1} B_{1}=12$ or $A_{1} C_{1}=12$.

When $A_{1} B_{1}=12$, $q=\frac{2}{3}$, and we get the three sides of $\triangle A_{1} B_{1} C_{1}$ as $12,18,27$;

When $A_{1} C_{1}=12$, $q=\frac{3}{2}$, and we get the three sides of $\triangle A_{1} B_{1} C_{1}$ as $\frac{16}{3}, 8,12$.
In addition, there is no third case.
Therefore, there are 2 such $\triangle A_{1} B_{1} C_{1}$.
Note: $\triangle A B C$ and $\triangle A_{1} B_{1} C_{1}$ have two sides and three angles, a total of five elements equal, but are not congruent.