Utility Session Management - It's Inside the Database in Teradata 13.10! (UPDATED) - comment by carrie

Unfortunately, a "large number of AMPs" is arbitrary.  If the number of sessions you would have to specify in order to get one session per AMP becomes unwieldly, too large a number of sessions for you to allocate to an ARC job (as it might with 500 AMPs, for example) then you could consider 500 a large number of AMPs.  In this context "large" means a number of AMPs that would require something outside of what is reasonable at  your site, in order to have one session per AMP for a single ARC job.  
 
ARC with one session per AMP will usually be more efficient since each session can only process one AMP at a time.   The alternative is to pick a number of streams that divides evenly by the number of AMPs, so each stream is doing the same amount of work.   I cannot tell you whether the default session management rule number of sessions would perform better on your platform than a number of sessions that you calculate yourself...you would have to try that out.  But it's always preferable if the math comes out even, based on the number of AMPs:  # AMPs / #AMPs per session / # streams.
 
I don't have any advice on number of streams.  You could post that question on Teradata Forum where it would reach a larger audience and see what others are dong.  If you are archiving to tape you are probably limited to one stream per tape drive.  You could start calculating a stream number based on even AMP distribution and then see how that number fits into how many storage devices are available.
 
Thanks, - Carrie