Capacity planning is the process of predicting a future system load in order to to determine the type of hardware required for a software.
Capacity = the number of agents (m) = the number of parallel process
The number of users logged into the system is not the concurrency one who are the actual users processing /querying. While there is an exception to every rule, you will rarely see concurrency go above 20% of the numbers of users unless a specific period of time such for a specific processes such as a financial close period.
Capacity planning is closed linked to Performance analysis. The difference is that: