1 or more logical volumes ?

This is my first NAS experience :
I have 8x 2TB disks and Open-e.
It will be used for daily backups for a group of 20 users and for archiving older data.
Clients will use smb, nfs and afp.

I choose for a RAID 5 setup, so I have 14 TB netto.
How many logical volumes should I create ?
Do I make 1 big volume of 14 TB or different smaller ones ?
Does it have an impact on efficiency or speed for the user ?

thanks

It wont matter much to performance with NFS.
Whatever makes it easier for you to manage ...