Peer Connections are counted by the number of unique peer ids that are created in a session. For each participant in a room, a unique peer id is generated. 

So for example, two people talking on a video call for any length of time would be measured as 2 peer connections.  If five people were on the video call, it would be counted as 5 peer connections.

Other articles you may be interested in: How is TURN Bandwidth measured and billed?| How is MCU/SFU Bandwidth measured and billed?| What happens when I exceed the included amount of Peer Connections or Bandwidth in my plan?