Question: I plan a new fileserver with 20 concurrent users and 2 TB of online storage. I plan to divide this data volume over 3-4 separate disks. I plan to buy 10k rpm disks. Implementing RAID-1, this would mean that i need 6-8 diks.
Should i go for sas disks or would sata disks be sufficient?

Answer: 
SAS drives are your best approach. Use enterprise SAS disks for speed and nearline SAS disks for capacity. 

There's no reason to buy SATA drives these days if you have a choice (unless they're SSDs). Can you elaborate on the server hardware make/model, RAID controller, etc.
SATA - no need to go SAS. Heck, if you have a decent Raid controller you do not even need 10k RPM discs - 5400 RPM discs are fine, with some SSD as caches.

This smells like you could use one of the SUperMicro storage cases - 24 x 2.5" front loaded SAS slots (+2 back loaded for the IS), a Adaptec 71605Q and 2 smaller SSD as cache - this is what I run now. Slow? Weeeellll..... Raid 6 over 8 discs and I am copying files around with 100+ mb/s - sometimes more than 500. THe SSD cache make a world of difference.

SATA generally is enough for that - remember, this is STILL low usage.

How do you arrive at the need to separate the volumes? Seriously? 2tb is small.