If you have purchased licenses based on traditional license model.
Otherwise, if you have capacity based license, you don't have to unless amount of data backed up exceeds terabytes allowed by your license.

you mean i have to purchase licenses for master servers as well?? but master servers are in the same cluster so i think i can use same license for the same cluster nodes??

License is needed for clusrer nodes altough those are inactive or standby state unless the node is deployed in cold DR site. Nodes in Cold DR site is subject of Cold DR license policy which allows users to use one additional copy of license.
Otherwise, if you deployed standby nodes in same site, I believe these nodes are not subject of Cold DR license policy.

kindly make it simple please i can not understand your point, please help me i am repeating my question:

two master server are clustered using microsoft clustering now in need to add one more master server in same cluster so do need need any more licenses or i can use same license key all server  are on the same site

You have to purchase additional license for new additional node if new node resides in site same with exsiting nodes.
License policy for cluster nodes is detailed on license guide which is provided for parters onParterNet.
You are 'accredited' user, you have access to PartNet. Check it.

NBU license could be based on

• number of host
• traffic flow (lan or san)
• processor could be considered as well

Each License is limited to Certain number of Master, Media and Client Server.

And Capacity based License is also available based on the data you backup.