Hi Krishna,
Thank you for the questions, its not a simple process to understand how many users can SSA handle. It depends on a number of cores, what the users are doing and what requests are being made. If we take each question:
What kind of Hardware size that I should suggest if the client wants to have unlimited users? The statement above would be true for this question. However, we have customers that have deployed the recommended server specification and have 300 - 400 users (not all concurrent) accessing the system, without any issue. My suggestion would be to ask the client what the users plan to do with the system, in order to get a better understanding of the request type.
What kind of Hardware size that I should suggest if the client wants to have unlimited users? Again without having a better understanding on what the users plan to do with the system it is impossible to suggest a recommended sever specification. However, this link should provide you with the recommended server specification: http://www.pitneybowes.com/us/support/apps/spatial-analyst-guides-for-spectrum-technology-platform.html
I would still suggest that you seek clarification from the client on expected usage.
What is the breaking point for SSA in terms of users and Data? From a data perspective it shouldn't matter how much data SSA presents, SSA doesn't actually load or hold the data, all it does is reference the location. We have a demo site that connects to over 500gb of data in one map configuration. We do have some best practice guides that provide how best to keep SSA performance high.
My suggestion is that you speak with the client to get a better understanding of how they plan to use the system, once you have this information, please contact your partner manager and we can better assist you in this matter.
Regards
Andy