Some one can tell me if know about companies (big ones) than use Asterisk how their principal solution for a call center, I mean huge, 500 members or more, another question is about the capacity of one server whith asterisk, how many lines can manage using a predictive dialer.[/b]
All very good questions. As for case studies, this is the only real documented ones that you may reference:
There are folks out there using it with hundreds of users+, but one area where Digium needs to improve is documenting this usage and making it available to the community. This is not easy, as many customers just download and use and Digium never knows about it. They have tried to run campaigns to give away cash for folks willing to document their usage, but have not seen any direct results from that today.
As for sizing, that is a loaded question and wholly dependent on how you plan to deploy, calling patterns, applications to run on top of Asterisk, number of simultaneous calls, etc.