Import materials is the act of the FBX importer importing, creating, and hooking up material assets for you. Still, each triangle has a material ID which is used to batch triangles together when drawing. Triangles with different material id’s cannot be batched since they would use a different shader. You cannot have more than 64 of these ID’s in your mesh. If you have more than 64 materials on your mesh in maya/max then that would do it. The reason for the limitation is to reduce draw call count.