A LUN is a logical section of storage. A LUN can be backed by a single disk or multiple disks. It can also be allocated from a disk pool/volume/aggregate depending on the storage vendor's terminology. A datastore is a description VMware uses for an area of storage that virtual machines can reside on. This includes NFS mounts and does not relate to whether the storage is on a LUN or some other block device, it purely refers to a place where a VM can reside.
It is either unsupported, or highly not recommended to put multiple VMFS partitions on a single block storage device (ie. what everyone refers to as a LUN), I can't remember off the top of my head, but just don't do it. You can however have a single datastore comprised of multiple block storage devices (ie. extents). With VMFS3 you could have a datastore larger than 2TB by adding extents, the 2TB less 512b limit for file size still applies though. With VMFS5 and the move to GPT, larger than 2TB block devices are now supported so there is no logical reason I can think of why someone would want to build a datastore using extents. There is no advantage but if one of the block device extents is lost, the entire datastore is lost.
VMware has a 256 LUN limit, yes. It is possible to have 256 LUN's and then NFS targets as datastores on a host. I am not sure whether there is a maximum supported datastore value for vSphere, somehow I would imagine 256 (block storage) + 32 (? for NFS, or whatever the max NFS mounts are).
Regards,
Corey