Load loading and performance testing is the only way to evaluate performance metrics and application instance requirements. You will need to define "concurrent users" - does this mean 25,000 concurrent transactions or does it just mean 25,000 active sessions? And if the latter, how often does the user visit web pages (for example, thought time between pages)? Then there are all the other moving parts: databases, Azure storage, external web services, in-role communication, etc. All of these steps in your processing pipeline can be a bottleneck.
Remember SLA: Suppose you can support 25,000 concurrent sessions (not transactions per second), what is the acceptable round-trip time? Two seconds? Five?
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