I want to run my software with Pm2 cluster mode with a single Cassandra instance. When the Node.js instances will start then every single instance will create a Cassandra connection pool.
I think this will cause problems.
What would be the best practice in this case?
This isn't going to be a problem at all. Cassandra is designed to handle connections from multiple clients.
In fact, it is best practice to have multiple client/app instances instead of just one so you can maximise the throughput of the cluster. Cheers!