Dss look for how much your RAID use , for ex: if you had 20TB HDD but you create a RAID 1 - then you will get 10TB to store on ( the other 10TB is Mirror!)
so in this case you need 10TB license .
but the same HDD if you for example use them in RAID 5 , then you will have about 14TB to store, then you need 14TB license.

and i'm with enealDC, its better to get what you are thought you will use in early future.

also as i know , you can update your license just by adding another license to what you have. for ex if you have 4TB ,and you want to store 12TB in future , then you can buy a 8TB license (you have 4 + the new 8 = the 12 ) and add this key to your system. thats all.