Consider I have 2 websites A and B without CMS. I want to host both websites in different custom domain. I am buying a site plan for $14/mo and hosting the website A. Now for website B, whether I have to again buy a new plan or else I can host in the old plan itself.
If hosting within Webflow then you will need to purchase a site plan per website. If you have any external hosting accounts like Hostgator or 20i then you can export your webflow site to your other hosting area.
Myself at WisdomainUK offer hosting solutions, so if you are interested then message me directly and we can discuss your requirements.