Bitwarden does offer a free 2 person organization. As I understand though this is only for the SaaS cloud version, and the self-hosted Bitwarden does not allow for a free 2-person organization. You can still have multiple individual users with a personal vault in your self-hosted instance for free without issue. It appears having a free 2 person organization has been extended to self-hosted servers. https://bitwarden.com/help/about-bitwarden-plans/
It seems access to both the API and subsequently the directory connector require a licensed business plan, either on a teams or enterprise plan.
Directory Connector functionality is available to Teams and Enterprise Organizations. To use Directory Connector, you must have access to your Organization API Key which can only be retrieved by an Organization Owner and securely shared using Bitwarden Send.
Access to the Bitwarden Public API is available customers on the following plans, Classic 2019 Enterprise Organizations, current Enterprise Organizations, and current Teams Organizations. For more information, see About Bitwarden Plans.
Conceivably you could have as many users as your hardware will allow, though to get full features for your users you will need to apply a license for these users.