There are limits on the number of domains you can monitor under a single subscription. These limits are based on how many of your domains have more than 10 breached email addresses, not the total number of domains you own.
For example, if you have 10 domains but only 3 of them exceed 10 breached addresses, you only need a Core 2 subscription, as it allows you to search up to 3 such domains.
For more details on how our subscription model works, see our FAQ article: What subscription size do I need to search my domain?