You can present up to 256 luns to an ESXi host. ( http://www.vmware.com/pdf/vsphere5/r51/vsphere-51-configuration-maximums.pdf )
I just tried to summarize the storage stack in an vSphere environment in a blog article a few weeks ago. (I referenced to a bunch of good resources)
http://vxpertise.net/2013/01/esxi-and-its-storage-queues/
If you include a lun with the physical compatibility mode the scsi commands will go "untouched" (except of 1 command if I remember correctly) to the LUN. If you chose the virtual compatibility mode the scsi command will go through whole ESXi storage stack.
Since we have on the ESXi level a queue per lun, the raw device will have its own device queue to this lun.