This is the answer I got from support:
"Regarding your question how many restore streams are used if you backup using 4 streams, the answer is, it will use 4 streams for restore even though you used 8 streams to do the recovery.
The way RMAN works, you can back up a database using 8 streams, and restore/duplicate using the same 8 streams. RMAN does keep track of media usage, so if you backed up a database using 4 streams, and then issue a restore using 8 streams, CommVault will issue the RMAN command using 8 channels, but RMAN/Oracle will only use 4 of those 8 channels since only 4 streams were used for the backup."
So that concludes that you can only restore with the same amount of streams as you did use for backup.
Thank you all for your inputs.