Maximum Internal Raw Capacity 24 TB (6 TB HDD X 4) (Capacity may vary by RAID types) (See All Supported HDD)
You can put in 4x 6TB drives.
This would give you 18GB usable space in SHR (Synology Hybrid RAID)
Yup.
The same as RAID5
Have a play with this.
https://www.synology.com/en-us/support/RAID_calculator
If you used 2x6TB drives in SHR, you'd get 6TB space (one drive protection, same as RAID1).
SHR adjusts it's mode on the fly as you add drives to it.
Right now mine has a single 6TB drive (no protection), if I added another, it would turn itself into a RAID1. Very handy feature.
-I keep manual backups of my data for now, didn't want to take the financial hit of 3x6tb drives
To utilise both, you'll need a managed switch, then create a trunk (aka link aggregation), so the 2x ports appear as one..
I went down that route, but tbh, it was a silly expense for a home user. I'd just stick to the one port.