When setting up a new datastore how many VMFS file systems should be created per LUN?

One datastore per LUN shoule be created using VMFS file system in VMware infrastructure

VMFS is the file system of your VMware Datastore so you can publish a large chunk of space from the storage and split it as multiple Datastore. lets say you have published a 5TB lun from the storage, that 5TB u can directly convert it in to a single Datastore or you can split it into multiple Datastors each Datastors will be individual vmfs.

VMFS-5, released with vSphere and ESXi 5.0, can handle volume sizes of up to 64TB and datastores of 2TB - 512B in size. VMFS is limited to 256 volumes per host and a maximum of 64 hosts can be associated with a single volume.