For domain searches, subscription sizes are based on the total number of breached email addresses on the domain across all non-spam list breaches presently in HIBP and are priced as follows:
If an insufficiently sized subscription is purchased and the domain exceeds the maximum number of breached addresses, an upgrade can be purchased and the remaining value of the existing subscription will proration against the new one, thus reducing the price. You can purchase an upgrade via the domain search dashboard.
If you have multiple domains, only the one subscription is required so long as it is sufficiently sized to query your largest domain.
Each subscription also provides the ability to query both domains and email addresses using the API. See the API dashboard page for more information.
When purchasing a subscription from the domain search dashboard, the recommended subscription size sufficient to query the domains presently being monitored is highlighted with a large red "Recommended 👍" banner: