9. A gardener is preparing to plant a row of 20 trees, with two types of trees available: maple trees or sycamore trees. The number of trees between any two maple trees (not including these two maple trees) cannot be equal to 3. Therefore, the maximum number of maple trees among the 20 trees is $\qquad$ trees.
【Answer】 12
【Solution】In any continuous eight trees, once a maple tree is planted, it means that another position can only plant a sycamore tree. We use the following diagram to illustrate, using — to represent a maple tree and ○ to represent a sycamore tree. Once the second position is planted with a maple tree, position $A$ must plant a sycamore tree. Regardless of where the maple tree appears, there is always a corresponding position that can only plant a sycamore tree, so in eight consecutive trees, there can be at most four maple trees. According to the previous derivation, in the first 16 trees of 20 trees, there are at most 8 maple trees, so the total number of maple trees is at most $8+4=12$. We can plant as follows: