This really depends on your workload. Start small and test performance to see how your containers do. Monitor CPU and memory resource usage, and then add cores or memory based on the kind of processes that you deploy in the container.
Make sure also to check the resource availability for the region you are deploying in for the upper bounds on CPU cores and memory available per container group.
Note
A small amount of a container group's resources is used by the service's underlying infrastructure. Your containers will be able to access most but not all of the resources allocated to the group. For this reason, plan a small resource buffer when requesting resources for containers in the group.