The Wersi can host 4 VST's at once although that does not limit you to four VST sounds at once. If for example your VST is multitimbral and is 64 voice polyphonic, you could theoretically play many sounds from that VST up to the maximum polyphony. The Wersi is unique in that once VST sounds are loaded into the sound banks, it does not differentiate between VST, factory, AKAI, or other third party sounds, it just sees a VST sound as if it were a factory sound. Want to play a VST sound, just hit the appropriate sound bank your favorite VST sound is loaded into and its there. The Wersi can support more than 4 VST's but in OAS 7 the host for the VST's has four VST slots. There are ways to work around adding and using additional VST's but processor speed limits the number you can run effectively. Its reported that some user have run as many as 16 VST's at once using Cubase as the host.

The Mediastation can host far more VST's than the Wersi (up to 64 I believe) but that is all in theory. Which VST you use, how much processor power and RAM it needs, and the type of sound files you play will greatly diminish the number of VST's you can run at once. It doesn't matter if the VST's are playing back from a Wersi, Mediastation, NEKO, or your home computer, all are limited by processor power and RAM.

The Wersi OAS 7 upgrade from OAS 6 is around $700-$800 USD. If you have an older version of OAS you might need to upgrade some hardware therefore the upgrade would cost a bit more. The price is very reasonable considering you are getting a VST host, new sequencer, many new sounds, new styles, new FX, new Real Drums, a VST analog synth emulator, FM synth, Wavetable synth, improved B4 interface, and a much improved sampler. In fact all the original OAS 6 factory sounds have been resampled to utilize the new and improved sampling rate of the OAS 7 sampler. You get all that plus much more. Its definitely a major jump from OAS 6 and well worth the cost of upgrade.