Trying to workout best way to go

[Enquiries]

Trying to keep this short due to char limit

Was maybe looking at getting the "Synology DiskStation DS1821+ 8-Bay" it says "Maximum Single Volume Size is 108TB" Is the 108TB the usable space after formatting does that include the space you lose from having an array, if you had 8 16TB drives can you have all that usable storage on one volume how about 18TB, at what stage would you be forced to have 2 volumes?

Thanks