Depending on the host CPU speed and the number of audio streams per Plug-In, you can instantiate up to 32 mono or 16 stereo HTDM Plug-Ins. The number of Plug-Ins you can run is also practically limited by available memory, as many HTDM Plug-Ins require large amounts of additional RAM.
As an example, on a G4/450 (not the fastest machine out there now!), you can easily run three or four of these Plug-Ins without the host CPU's UI getting bogged down, or error messages.