web domain

A web domain is a realm of administrative autonomy, authority, or control on the Internet, the part of a web address which identifies an organization.

Yes, you can update the list, adding or removing domains at any time. There is no limit on the number of domains.
There is no limit on the number of specified domains. In case of the self-hosted webfonts, the total number of monthly page views, needs to cover all domains. For example, if you use ten different domains, and each has 100,000 monthly page views, you need a Self-hosted Webfont License for...