How many bays does the Synology DS1520+ NAS start with and how many can it scale to?

The [Synology DS](https://www.amazon.com/s?k=Synology+DS&tag=wehavethat-20)1520+ starts with 5 internal bays and can scale to 15 bays by adding two optional DX517 expansion units, without needing to swap the chassis.