Master server - only one master server per backup domain. More than one master is only for cluster environments when you need HA.
Media server - (moves data between client and storage unit(s)) number of media servers depends on the load i.e. number of backup jobs being performed, number of storage units, amount of data being backed up, etc.
NB: although Veritas recommends that number of media servers should be limited to less than 50, each additional media server should only be added if it is really required. Remember you (and sys admins) have to manage the backup domain too! More number of media servers = longer down time.
Performance and Tuning guide is a good reference.