Hi Aanmin83 and welcome to the forums!
There is a finite level to the amount of resources and IP phones that can be attached to any system.
With the CS1000 I believe each Signaling Server can service upwards of 5,000 IP phones, add another Signaling Server and you can serve a total of 10,000 IP phones. Those numbers may have changed slightly over the past few years but I did find a Nortel document that states the CS1000E can support upwards of 22,500 IP phones (11,250 SIP phones).
http://www.nortel.com/solutions/multimediacomm/collateral/nn121201.pdfGood Luck!