As of today, you can create 20 global secondary indexes per Amazon DynamoDB table in AWS GovCloud (US). If you need more than 20 global secondary indexes, you can request an increase.

from Recent Announcements: https://aws.amazon.com/about-aws/whats-new/2019/11/you-can-now-create-20-global-secondary-indexes-and-have-100-projected-index-attributes-per-amazon-dynamodb-table-in-aws-govcloud-us/