I think this is best practice:

  - whenever you need to follow backup windows and other restrictions - do it 
first
  - for the rest of the clients try to spread the workload over time and backup 
level. The NMC reports can help you to find out the right timing/schedule.

To the best of my knowledge you can have as many clients as you want in one 
group - of course they will most likely not all be processed simultaneously.
However, i would not add more than 20..25 to a group. Smaller groups are much 
easier to monitor and/or to restart if needed. 
Do not forget that a group must totally finish before one can restart it.

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