How many concurrent connections can a MERN real-time app handle?

A single Node.js Socket.io instance handles 10,000 to 50,000 concurrent connections depending on server resources and event frequency. For larger scale, Redis pub/sub as the Socket.io adapter allows horizontal scaling across multiple Node.js instances. PM2 cluster mode scales across CPU cores on a single server. We load test at 3x expected peak concurrent connections before every production launch.