http/security_context_cache_size is a SAP Parameter attribute which is used to control size (number of entries) of the HTTP security session context cache information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
If HTTP Security Session Management is activated (see transaction
SICF_SESSIONS), session contexts are saved in a local server cache, so
that session inactivity timeout monitoring can be performed.
You cannot change the cache size at runtime. It is calculated from the
retention period of the cache entries (that is, the session timeout
value) and the expected number of concurrent users of an application
See SAP Parameter Documentation for full SAP documentation for this profile parameter.