This page hosts some example IdP throughput numbers for some environments. Every environment will be different and this should be used as an estimate, not as a substitute for your own load tests.
AWS EC2
A single c1.medium instance running the IdP in Jetty without Terracotta and a million user records and a standard set of attributes in an LDAP directory with authentication randomly choosing one of them can handle roughly 65 transactions per second. Optimizations can raise this to 80 tps. Memory(roughly 50% of "physical" memory is used by java) and disk utilization(negligible) are not considerations even with hundreds of thousands of concurrent authentication sessions, but situations can arise where the garbage collection with limited total memory results in a freeze or crash of the jetty process. With stateless clustering, this will scale linearly. With stateful clustering, performance is likely to be considerably different. A single c1.xlarge instance running in the same fashion can achieve 300 transactions per second and is not susceptible to freezes or crashing during garbage collection.