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Hello,
Thank you for reaching out. As you properly stated, the Insufficient Capacity errors mean that at a given moment, AWS lacks capacity for on-demand Instances in an specific Availability Zone when you tried to start or launch an Instance (1).
On-Demand capacity fluctuates overtime, so waiting few minutes, changing Instance type temporary or (when launching an Instance) select a new Availability Zone where capacity is available. You can use the AWS CLI command describe-instance-type-offerings to validate the Instance types you can launch in an Availability Zone (2-3).
If your operation is critical, your team may consider also take a look to On-Demand Capacity Reservations so you have specific compute capacity allocated in specifics Availability Zones. Note that there is a cost associated with the use of Capacity Reservations (4-5).
Unfortunately, there is no roadmap as to how more resources are made available over time. As to which Instances are prone to be more likely to encounter this issue, it all depends on the resources available c5a.8xlarge does require 32 vCPUs. If you take that multiplied by x number of Instances, they add up for sure.
I understand you were looking for a more suitable answer, in AWS our customers are our priority and we constantly keep on adding more resources to alleviate this kind of issue from happening.
Have a nice day!!
References
(2) describe-instance-type-offerings
(3) Find an Amazon EC2 instance type - (Find an instance type using the AWS CLI
(4) On-Demand Capacity Reservations
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