How many heading tags are available for heading and subheading when using HTML?

There are six levels of heading tags available for structuring text hierarchically:

<h1> to <h6>.

<h1>: Main heading
<h2>: Subheading
<h3>: Sub-subheading
<h4>: Sub-sub-subheading
<h5>: Sub-sub-sub-subheading
<h6>: Sub-sub-sub-sub-subheading