Hello,
How are the ivrs implemented? 
* PHP client scripts? 
* Javascript routing script? 
* yaypm?
* Another way?
That is a big number of calls considering that the media passes through Yate.
I haven't really tested it with that many simultaneous ivrs. My suggestion is to develop the ivr and then add a proxy in front that sends the calls to multiple ivr servers. At a certain point you will have too many calls.