This is a very broad question and the answer is 'it depends', and here is why.
NDMP backup will use multiple streams in one policy if it is enabled in the policy properties AND the backup selection configured as follows:
.. and so on.
Now if you want to restore directoryB and C you can do that and they can be run as parallel jobs, provided resources are available - this is also controlled at the storage unit and storage server levels.
Also it may depend whether you use the Data Domain NDMP Tape Server or use NBU NDMP Tape Server with Data Domain OST back-end
You are going to want to talk to the NDMP vendor, as well as EMC ( for the data Domain ) - since the number of streams is limited by the NDMP filer and the data domain MORE than netbackup. Essentially NB allows you to run as many as you want, some filers are limited to 4 streams, and the data domain models may have limits as well.
From NB perspective you are limited by how many image files you have, and whether they are on tape or disk, since you cannot restore the same tape twice at the same time...