https://awscli.amazonaws.com/v2/documentation/api/latest/reference/ecr-public/describe-registries.html lists multiple registries for a public ECR registry (i.e. ecr-public). How to do the same for private registries
AWS ECR CLI doesn't mention describe-registries in the following section and the question is how to fetch multiple private registries if configured https://awscli.amazonaws.com/v2/documentation/api/latest/reference/ecr/index.html
Each private registry is belong to its AWS account. https://docs.aws.amazon.com/AmazonECR/latest/userguide/Registries.html
So you can only use describe-registry for your account.