Attaching multiple ENI's to a single AWS Instance can cause dual-homed instances which could increase network complexity and could introduce unwanted network paths leading to unauthorized access.
In AWS Console -
In Terraform -
References:
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-networking.html
https://registry.terraform.io/providers/hashicorp/aws/3.76.1/docs/resources/instance#network_interface