There is no limit to how many levels we can have, for either sane or insane headers!
HTML is randomly limited to h6, so OurBigBook just renders higher levels as an h6 with a data-level attribute to indicate the actual level for possible CSS styling:
<h6 data-level="7">My title</h6>
The recommended style is to use insane headers up to h6, and then move to sane one for higher levels though, otherwise it becomes very hard to count the = signs.
To avoid this, we considered making the insane syntax be instead:
= 1 My h1
= 2 My h2
= 3 My h3
but it just didn't feel as good, and is a bit harder to type than just smashing = n times for lower levels, which is the most common use case. So we just copied markdown.