you can use metrics chargeback trend report to see the growth by CommCell, Clients, Agents, Storage Policies. Each of these entities can be further drilled down to Client, Agent, Subclient, Storage Policy, Storage policy copy.
we have data for last 24 months if you use private Metrics server.
if you have enabled chargeback data to be uploaded to cloud.commvault.com, we will show trend for the last 12 months.
Documentaion : http://documentation.commvault.com/commvault/v11/article?p=38148.htm