As many as you can afford. Seriously, you require only one(1) license per domain - regardless of the number of users being tracked. You are allowed to run as many instances of Password Sentry on the same domain, providing that each instance (e.g., directory or subdomain) is under your ownership and control.
Please note that typically you only need to install Password Sentry once on a given domain. You can create individual setups within a given Password Sentry installation where each setup monitors a separate location (each with its own user file or database).
Last Revised: 2015-04-29 20:24:03