Are you planning to not use any RAID protection for your data? Or plan on exactly hallving your data capacity using RAID I?
If you purchased 2x6TB disks and placed them into service using JBOD ( disaster waiting to happen to your data ) you would have 12TB of unprotected data.
If you used the same 2x6TB disks to create a RAID 1 mirror the exact same data would be copied to both disks leaving you with 6TB of usable space. You could in the future buy the next set of 6TB disks and have a total of 12TB of space.
…Or you could pony up and buy the 24TB model now and run all four disks in RAID 5 with a little over of 18TB of protected data.
…If i was in your shoes and needed to scale to 24TB of protected space I would be looking for higher end NAS product like a Synology 8-disks base unit that allows for expansion drive bays.