Recently, when running terraform apply, my script has been failing intermittently when provisioning around 90 ec2 instances.
The root cause of my script failing was due to raising the parallelism count beyond the default ( 10 ). When tracing the debug logs, i noticed a lot of API requests to AWS that terraform made has failed with RequestLimitExceeded, after setting back to default, i am able to consistently provision
200 ec2 instances without failure.
There are more discussion about this issue on github here.