Frustum culling

How unity culling object by frustum, example: I have one object consisting of 10 meshes, will it cull every mesh or whole object only?

If, by "object" you mean a single GameObject whose Mesh has a number of SubMeshes, then I think it's done by whole-object only, rather than sub-meshes. This is backed up by the fact that there appears to be only one function to Recalculate the Bounding Volume of your mesh as a whole, not individual sub-meshes.

However, if by "object" you mean a parent GameObject, with multiple child GameObjects, each with their own mesh renderer, then I believe each GameObject will be view-frustum-culled independently.