Scaling MySites; How Many MySites per Content DB?

Scale-out
The primary scale-out strategy is to use multiple content databases in the MySite web application. To meet the guideline of 50,000 site collections per DB, you should plan a minimum of 2 content databases to handle 100,000 MySites.

Scale-up
To further ensure the overall performance of SharePoint when using multiple content databases, configure SQL Server so that the databases are hosted on different physical drives (spindles). This is usually possible with SAN storage solutions also. My instinct is that you'd want to balance this with the scale-out strategies and usage patterns. If you had 200,000 users with MySites and a large portion really began to use MySites heavily, then you might want to use 20 content databases distributed across 5 or 10 physical drives.

Scaling MySites; How Many MySites per Content DB?