How do I set the maximum number of streams for multiplexing?

From Backup Central

You must set multiplexing for both the storage unit as well as the schedule. The storage unit can have between 1 and 32 streams per drive. When adding a storage unit specify whether or not multiplexing will occur, and set a value from 2 to 32 for each drive if using it. You also need to specify the number of streams allowed per drive for each schedule for which you wish to use MPX.