This is really depends on how you configure disk library but I would say 1.5 per disk stream so if you have 8+1 RAID 5 disk is available, a number of stream should be up to 8 stream per target library.
Multiplexing is depends on how fast destiantion device to be able to handle.
Often too many multiplexing jobs causes exhaustion of memory or I/O error because it requires memory reservation per stream. However, it would suggest to start from 4 as per storage policy to start tuning with.
Also there is a catch to use mutiplexing !
A golden rule is when your back up faster, restore time will be taking longer.
So check your RTO time and never backup DB files with multiplexing (Horrible to restore)
Hope that will help.