RAID types can be upgrade (RAID1 can go to RAID 5 or 6 etc) so even if you start with only 2 drives you will be able to upgrade. Also you have to take in consideration if you will use a conventional raid type (same disks with same capacity) or DSM SHR type (for mix size). ...
DSM8100-000$4.6562 CLL4729C TR BZT55B9V1 L1G 1N5953AP/TR8 JANS1N4623DUR-1/TR$449.6724 JANTX1N4489DUS$42.9700 JANTX1N5528DUR-1$40.9199 JANTXV1N4480US/TR$15.4200 1N5921CG$5.2400 1N4126UR$3.2900 1N5263BUR-1/TR$2.8800 SMBG5385A/TR13$1.9100 ...