Reflectional symmetry is the mirror image of one side of image to the other when we divide the image. In other words it is one half of image is the reflection of other half of image.
In isosceles trapezoid, the length of the sides are same . the bases are not same.
When we draw a vertical line in the middle of isosceles trapezoid, the left side of the image is the reflection of right side .
So there is only one line of reflection symmetry.