QueueMetrics is licensed based on:

• the number of agents it needs to track
• the number of PBX servers you want to monitor
• a specific duration

You need an agent for each separate agent entity you want to keep track of.
So, if you have two shifts of 20 persons each, you need 40 agents total, no matter how many of them are logged in at once.
If your agents change over time and you want to keep track of both present and past ones each using a different code for each, you will have to factor that in as well.
So, for most call centers, you will need something that matches the number of paychecks you write every month for call-center agents.