I have 20k elements my setup is 2 x 1.5TB each and 900+ IOPS each.
How many flow sources are you planning on monitoring?
6 disks for RAID 10 sounds about right. You need to make sure you have log and temp files on separate volumes, though
For that you'll need more that 6 disks total.
II'M sorry let me put a little more detail on my setup.
SQL standard 2008
2x 1.5 TB raid 10 For DB and temp.
12 disks each.
900+ IOPs very important for NTA.
log and all that other SQL Junk you can store in slow drives.