Things that limit migration processes...
1) migproc for the storage pool
2) number of drives in the atl
3) number of unique node's data in the diskpool you are migrating to tape

The smallest of these is what will determine how many drives are active...
So if you have 6 drives
and you "upd stg <somediskpool> migpr=6"
and you initiate a migration (drop the high & low down to 0)
AND you only have two (2) node's data in the diskpool, then only two
migration processes will start.
AND as soon as one of those node's data is fully migrated, there will only
be one process...

Dwight