You can now enable increased elastic network interface (ENI) densities for select Amazon EC2 instances running tasks that use the awsvpc network mode on Amazon Elastic Container Service (ECS). These enhancements can be enabled by opting in to an account setting on a per account or per IAM role basis.

from Recent Announcements https://aws.amazon.com/about-aws/whats-new/2019/06/Amazon-ECS-Improves-ENI-Density-Limits-for-awsvpc-Networking-Mode/