The free Cookiebot subscription is limited to 1 domain and 50 subpages or fewer (as described also on the pricing page of the Cookiebot website) - counted by the number of unique URLs on the website.
If you have several domains with 50 subpages or less, adding them to the same account, they will all require a premium subscription each. This is regardless of whether their combined total number of subpages is 50 or below. Subscriptions are per domain and per subdomain.
If you have one domain with 50 subpages or less and one or more domains with more than 50 subpages, adding them to the same account, they will all require a premium subscription each. It is not possible to have a free subscription on an account with other domains.
See How do I know what plan I've subscribed to? for instructions on how to see the number of subpages identified by the scanner - and the corresponding subscription size for your domains.