The BEST metric to demonstrate that servers are configured securely is the total number of servers:

Exceeding availability thresholds relates to system uptime and performance, not directly to security configuration.

Experiencing hardware failures relates to reliability and physical integrity, not specifically to the software configuration security of the server.

Exceeding current patching standards indicates a lack of up-to-date vulnerability remediation, which is a security concern, but "meeting the baseline for hardening" is a broader and more comprehensive measure of overall secure configuration.

A baseline for hardening specifies a set of secure configuration standards (e.g., disabled unnecessary services, restricted administrative access, secure protocols) that servers must meet to be considered secure. Measuring adherence to this baseline directly demonstrates that servers are configured securely according to established best practices.