Is it possible to create more heading styles, like h7, h8, and so on?
Is it possible to have more than six different types of headings on a website?
You can create any element you want on a page, but I think your question is whether it will be interpreted correctly by a user agent like a browser. Browsers will allow you to create, apply styles to, and access arbitrarily-created elements on the DOM regardless of whether they conform to the spec implied by your declared DOCTYPE.
However, in general, it's best to avoid creating deeply nested structures; users find such hierarchies difficult to follow. In addition, as other answers linked in the comments have pointed out, there may be unintended consequences with page accessibility, SEO, or script compatibility. I have yet to find a use case that actually needs such deeply nested hierarchies, except when posting long legal documents as a single HTML page.