Just one, I would think and judging by the size it would only be a 2.5" drive as well and a quick google search could not locate a 200GB one so you may well be limited to the 120GB listed on the Apple site.
There are a couple of FW/USB enclosures that fit perfectly below the Mac mini itself. There you can use a normal 3.5" drive (500 GB for example) and use that as the startup drive as well. No problemo.
