You can only create one user or organization site for each GitHub account. Project sites, whether owned by an organization or a user account, are unlimited.

There are three types of GitHub Pages sites:
project, user, and organization. Project sites are connected to a specific project hosted on GitHub, such as a JavaScript library or a recipe collection. User and organization sites are connected to a specific GitHub account.

To publish a user site, you must create a repository owned by your user account that's named <user>.github.io. To publish an organization site, you must create a repository owned by an organization that's named <organization>.github.io. Unless you're using a custom domain, user and organization sites are available at http(s)://<username>.github.io or http(s)://<organization>.github.io.

The source files for a project site are stored in the same repository as their project. Unless you're using a custom domain, project sites are available at http(s)://<user>.github.io/<repository> or http(s)://<organization>.github.io/<repository>.

The publishing source for your GitHub Pages site is the branch and folder where the source files for your site are stored. If the default publishing source exists in your repository, GitHub Pages will automatically publish a site from that source. The default publishing source for user and organization sites is the root of the default branch for the repository. The default publishing source for project sites is the root of the gh-pages branch.