Usage pattern
EC2 flex instances are a great fit for when you don’t need to fully utilize all compute resources.
You can achieve 5 percent better price performance and 5 percent lower prices due to efficient use of compute resources. Typically, this is a great fit for most applications, so C7i-flex instances should be the first choice for compute-intensive workloads.
However, if your application requires continuous high CPU usage, then you should use C7i instances instead. They are likely more suitable for workloads such as batch processing, distributed analytics, high performance computing (HPC), ad serving, highly scalable multiplayer gaming, and video encoding.
Instance sizes
C7i-flex instances offer the most common sizes used by a majority of workloads going up to a maximum of 8xlarge in size.
If you need higher specs, then you should look into the large C7i instances, which include 12xlarge, 16xlarge, 24xlarge, 48xlarge and two bare metal options with metal-24xl and metal-48xl sizes.