Connections you see in the Atlas Metrics represent a total of connections caused by:
Heartbeats and replication between the nodes (20-25 connections)
Agent connections (backup, monitoring, and automation agents)
Driver connections
App connections
Every new cluster may have more than 20 connections from the moment of its creation, before it is even being used to store data.
For M10+ clusters, if you'd like additional information regarding your cluster's active connections, you may run the following script (via the mongo shell) to generate a basic list of all active client connections:
db.currentOp(true).inprog.forEach( function(op) { if(op.active = true && op.client != undefined) { var miniOp = { client : op.client, description : op.desc }; printjson(miniOp); } } );
Want to know more? Check the documentation on Atlas connections here.