Number of devices in the target segment

As described in “Worker process requirements for parallel sorts”, the number of devices in the target segment determines the number of consumers for sort operations, except for creating a clustered index on a partitioned table.

Performance considerations for query processing, such as the improvements in I/O when indexes are on separate devices from the data are more important in determining your device allocations and object placement than sort requirements.

If your worktable sorts are large enough to require parallel sorts, multiple devices in the system segment of tempdb will speed these sorts, as well as increase I/O parallelism while rows are being inserted into the worktable.