Ideally there is no limitation.
Starting Ops-Mgr version 3.8.1 we fixed issues some performance issues due to groups..
But the sizing is only done for 250 in DFM 4.0
Check the sizing for more info.
http://media.netapp.com/documents/tr-3440.pdf
But 1000 groups shouldn’t be a problem.And theoretically there is no limitation.
3.8.1 or latter versions of DFM you shouldn’t have problems with 1000 groups.
Regards
adai