How to Update and Manage Millions of Databases

We are looking to shard our application, so that each customer has their own database. The data contents of all databases are different, but the structure is the same for all customers. Imagine there are 2 million customers. How do we update 2 million databases to: 1. Create, modify or eliminate columns. 2. Create, modify or eliminate tables. How do we do this across millions of databases across dozens of instances? We know how to do it manually, but how can this process be automated? So that we just modify one instance, and that propagates to all customers. Remember each customer has completely different data in each database. Thanks!