There are really two questions here, since "paying for a domain" really involves both registering the domain and hosting the domain (and any subdomains).
As to the first question, the registrar will only charge for registering the domain (
example.com) and does not charge for subdomains. So your $10-ish per year fee for the domain itself covers an infinite number of subdomains.
As to the second, it varies by host. Some hosts do not charge extra for subdomains, while others do. Yet others don't directly charge for subdomains but do have limits on how many websites you can set up. So, check with your host, or look for one that enables you to set up as many subdomains as you need.
Finally, also keep in mind that if you want to add an SSL certificate to your site to make it more secure, you'll either need a wildcard certificate (covers every subdomain of
example.com) or a separate certificate for every subdomain (
two.example.com are all separate). As long as your host supports Let's Encrypt certificates and doesn't charge for installation that shouldn't be much of an issue, but since some hosts do charge it's something to consider.