I've never had any troubles running a few of these at a time, although I haven't ever had a need to use more than three or four at a time. While I no longer use this plug-in in my set up, I used it for a good while in Cubase SX that I ran with a Hewlett Packard lap top that had a 3.0 Ghz processor and 2 GB of RAM. Since Steinberg makes both this plug-in and Cubase SX, the compatibility was great and it always ran quite smoothly for me.
While I liked using this plug-in a lot, I haven't used it since I got my Pro Tools rig because there isn't a version of this plug-in that can be used in Pro Tools. This being said, if you are running a Steinberg based sequencer like Cubase or Nuendo, I would definitely recommend looking into Steinberg Q-Metric because it is a good sounding plug-in that is easy to use and compatible with these pieces of software. However, if you have another sequencer of Pro Tools, I would look elsewhere as I don't know how well this would transfer to another type of software if it would at all...