Depends on which product you buy. Three rows got me two feet.
http://www.bunchobikes.com/pond42.jpg

Larry wrote:

I haven't put them down on concrete but I did knock the lips off my 1'
wall just so I wouldn't have a little more setback on each row.
You're going to have go up 6 rows to get 2'. As long as you get the
bottom layer level and have plenty of backfill, what's the problem? I
used a 3" chisel and hammer on mine.

The Tilton Family wrote:

I purchased two pallets of castlewall blocks to build a two foot
high retaining wall to replace a railroad tie wall that had gotten
rotted and ugly. I then found that removing the ties was a major
problem and was wondering if I could successfully build the wall just
in front of the ties, which would require the base course be on the
concrete driveway. That would entail chiseling the lips off that
base course. Has anyone had any experience in laying these blocks on
a concrete base rather than sand?
As Always Thanks,
Bob T