How do I handle scaling to thousands of concurrent sessions?

Our WebSync Cloud environment handles 10,000+ concurrent sessions. To provide this level of concurrency we're using legacy c3.xlarge AWS instances, the modern equivalent would be c5.xlarge. We scale our WebSync Cloud environment with 4 servers running full-time behind a load balancer, with a single cache service running in the backend synchronizing them. Details on the cache provider are available here.