This depends on the banner you want to show on subdomains. If it is the same banner as on the parent domain (including the same cookie report), we have a special option that will allow it to work on any number of subdomains. In this case, you have an unlimited number of subdomains supported.
If you want to show a different banner on every subdomain (with different cookie reports), you will need to add each subdomain as a separate website. In this case, every subdomain will be counted as a separate website.
Note that only active domains are calculated. This means if you are reaching the domain limit and you have some unused websites, you can disable cookie banners for those unused websites to create new cookie banners.