Hi, this is a hardcoded limitation in the shader file Engine\Shaders\LandscapeGrassWeight.usf. It renders multiple layers in multiple passes, and each pass can do 4 layer except the first in which the first two values are used for the heightmap.
The original shader file only allowed for 3 passes, which gave 2 layers in the first pass and 4 in the subsequent two passes for a total of 10 layers.
The attached file supports two more passes, so this expands the number of supported grass layers to 18. Just replace the shader with the updated version. You can keep modifying the file yourself if you want more than that.
The attached shader is valid for 4.9 and 4.10.