In my 15 year career? Dozens. Maybe low hundreds. Depends what you work on. Oracle is not making any friends lately and a ton of companies a whole-sale migrating to Postgres, MongoDB, DynamoDB or some of the NewSQL contenders. It’s like 50% of the projects I’m involved in. Results are generally positive, with some spectacular wins (x3000 acceleration or x1000 lower costs) and a few losses.