Just looked to their self hosted plans:

    - No limitation on search, members, etc.
    - 10 user limit for mobile notifications, can be relaxed via community (for non-profits, FOSS projects, etc.)
    - SAML/LDAP *support* is available, you can configure it. They won't provide answers to your questions.
    - Actually, all Zulip features are enabled sans Mobile Notifications, but for most of them, you're on your own. If you know what you're doing, it's not a problem, I assume.

IOW, for self-hosted plans, you pay for support, not the software. a-la early RedHat model.

Ref: https://zulip.com/plans/#self-hosted-sponsorships