Why can't there be more than 6 headers ?

Answer 1

Because, usually you don't need more. Actually, a good page layout doesn't need more than 4 headers. Beyond, your reader is lost.

Answer 2

I think using any more than three headers is bad practice unless it's part of a script's function... which is very rare to see.
My guess is that they didn't think it was necessary to have more than six header sizes. I believe there should only be <h> so you can adjust the header size accordingly.

Answer 3

i think use than nuber example
<h1>
<h2>
<h3>
and
.....