Hello Spalding
You can only use 4 VST Plug-ins simultaneously, Such As;
B4 (approx 110 sounds + User, and free use as if it was a B3 Organ)
Pro 53 (Approx 100 sounds + User, and free to use as a Prophet Synthesizer)
Kontakt 2 (Sample Player/Editor) (Number of sounds limited by available Ram)
The Grand (Steinway and other Top Piano samples)
If you wish to use another then you have to replace one that is already there, however I believe (I am on OAS 6 so not totally sure on OAS 7) they can be stored in Presets, so that when you select the Preset the appropriate VST Plug-ins are loaded automatically. (There are also VST Hosts that are themselves Plug-ins, so if you understand what you are doing, more then 4 can be loaded)
VST Plug-ins were originally designed to be used in studios, and the engineers new what VST Plug-ins could be combined and not overload the computer systems, however as Arranger/Organ players are not studio/computer engineers, Wersi put a safe limit on the number of VST Plug-ins that could be used simultaneously so that that the users do not run into problems. (As faster CPUs and Ram drop in price and become the standard in the instruments, then Wersi will increase the number of VSTs allowed, but always making sure that they will not cause problems, even to a beginner)
This is one of the advantages of Wersi. In that there OS is designed even for people who have never played an Arranger/Organ before.
As an Example
If you went to a friend who had one, and he had loaded Akai samples, VST Plug-ins etc.you would find it no different to playing your PA1X, but the sound quality would be just like you hear in films, or in live performances.
Hope this helps

Bill