Is it possible to run multiple docker containers in one EC2 instance through AWS ECS (EC2 Container Service)?
Yes.
AWS's documentation/product details doesn't ever come out and say it explictly, but it talks about launching many containers to a cluster. A cluster can be one instance.
When configuring a container, you specify memory and CPU usage. ECS uses that to "schedule" (or "pack") an EC2 with Docker containers.