if the backup speeds are fine and the overall resources on your existing servers are not fully utilized, you don't need to add additional servers. Usually, you add another media server, to improve backup performance - but this may also be achieved by changing existing servers (like adding 10Gbit Ethernet).

I see thanks for the information Sir

I know what to expect from my media servers, when the media server pool reaches their maximum, I add yet another media server. I alwyas allow some slack in both end, as network connections and storage unit group does not balance 100%

HP-UX with ether channels                          : 6-8TB/24h
Linux media serervs  with 10Gb ethernet : 10-15TB/24

I see but is there a limit number of client that will be reporting to the media server or none?thanks

That depends probably on the way you configure the netbackup port usage - if you define a small client reserved/client port window you might run out of ports for the network connections of your media server, if too many jobs run concurrently. So you might run into trouble somewhere between 500-1000 concurrently active jobs on one media server - but then you will run into the previously mentioned things like slow backup performance. Assuming you run 500 Jobs over a 10 Gbit Line (Assuming 800 MB/s throughput, which you may or may not achieve...) you will have something like 1,6 MB/s per running backup job - and you do not want that. So the answer is like "there is no limit you will reach if you have a look at the performance".