Set "Limit jobs per policy" in the policy for the server. All streams from the server will start, but only the configured number in "Limit per jobs policy" will go active. 

Limit jobs per policy

http://www.symantec.com/docs/HOWTO34477

I agree with Nicolai - also, if you disable allow multiple streams in the policy, you will only send one steam, when then can only go to one drive, so this would work as a solution also.

Martin

There is a good section in older versions of NBU Admin Guide II explaining Multiplexing, multistreaming policy limits, etc and how jobs are assigned to drives. For some or other reason I cannot find this section in 7.1 manuals, although still very relevant.

See chapter 1 of NBU 6.5 Admin Guide II for Windows http://www.symantec.com/docs/TECH52804

Also, if you have lots of other clients and policies running you can just limit the number of jobs for the client itself
In that way you can leave its policy(ies) unlimited but restrict how many go active for it
Just add it to the Master servers host properties - Client attributes section and set how many jobs it is able to do
In that way if it has 2 policies with 20 streams in each it will limit it to running say 5 jobs at any one time
As soon as one stream finishes another stream will be able to go active
Hope this helps

I love NetBackup ...so many differnet ways to do exactly the same thing ....

Don't forget about MPX value, if you MPX 4 it will put 4 streams on the same drive. So if you limit jobs per policy to 8 and MPX at 4, two drives will be used.