One license should keep your customers covered. Localhost, staging and development environments won’t get counted toward the license’s maximum allowed sites if its domain name is clearly a dev or staging site.
TLDs that are considered as dev or staging:
Subdomains that are considered as dev or staging:
stagingN.* (SiteGround; N is an unsigned int)
*.wpengine.com (WP Engine)
Additionally, if your domain is localhost (with any port), it will also be treated as a localhost domain.
If your customer want to utilize a license on a dev site that doesn’t fit the criteria listed above they can deactivate the license from within the Account page in their WP Admin dashboard, and then reuse the license.