Unlike hard-bounced contacts, which are automatically excluded from your subscription's contact count, unsubscribed contacts generally do still count towards your Zoho Campaigns subscription plan limit, since the record itself remains valid, it's just opted out of receiving marketing.
By default, clicking Unsubscribe only removes a contact from the specific list or topic the email came from. They can still be part of other lists in your account, and would need to unsubscribe from all communication, or you would need to mark them accordingly, to stop appearing everywhere.
Zoho Campaigns has a separate "non-marketing" contact status, available on paid subscriptions, for contacts you want to keep on record without them counting towards your subscription's contact limit. An organisation can hold up to 1 million non-marketing contacts. Marking a contact as non-marketing stops them receiving any email campaigns and unsubscribes them from every topic, though you can mark them as marketing again later if needed.