we are running Windows Server 2008 R2 64 Bit, what is your system?
By means of a registry change on the server, number of max. simultanous QVB.exes can be increased.
The change is described in this thread:
http://community.qlik.com/message/203910#203910 at bottom, (SharedSection=1024,20480,2048 instead SharedSection=1024,20480,768)
We have applied this change on our server, with very good experience!
However, note that every change in the server registry must be done very carefully and properly prepared & analysed.
You will have to re-setup your server if you enter any wrong values, because of possible crash at next system startup.
Regards - Marcel