Is it possible to create html tags h7, h8, h9 and so on

The HTML specification defines six headings. Browsers recognize six headings. If you start using <h7> and so on, then you'll be using an invalid element.

If you find yourself needing an <h7> then you should probably take a look at how you're structuring your site. Not every single title deserves a heading element.

Generally speaking, you can create as many headings as you want, but they must be defined as classes. So if you are looking at making more than six headings for organization purposes do something like:

.h7{
    color: #111111;
    font-size: 16px;
    text-transform: uppercase;
 }

<p class="h7"> I'm an extra heading </p>

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.