RAID 5:
Setting up several disks to all work as one with all data being written on 2 disks. The sky is only limited by the number of drive headers in the controller and the size of the disk. All disks must be the same size or all will be treated as the smallest disk. Total amount of storage is SIZE_OF_DISK * (NUMBER_OF DRIVES -1). So if you put in 8 500 Gig drives, you'd get 500 * (8-1) drives = 3.5 TB. 8 TB drives would give you 7 TB. 4 500's would give you 1.5 TB.
You do need a controller chip/board with RAID 5 capability. Many today only have JBOD, RAIDs 0 and 1 - and maybe 10.
You can lose a drive and lose nothing. Swap in a new one and let it rebuild itself.
Bryan