It is a recommendation to have just one per page, even validators print out a warming if there are more, but other recommendations contradict it like:

1. Any rank can be used as the heading of an explicitly-defined section, although this practice is not recommended.
2. Never skip heading order

and there is also the sectioning elements like aside which's outline shouldn't affect the page's outline.

It is pretty confusion, so at least I want to know what I'm gaining following the only one recommendation.

It is a recommendation to have just one per page, even validators print out a warming if there are more, but other recommendations contradict it like:

1. Any rank can be used as the heading of an explicitly-defined section, although this practice is not recommended.
2. Never skip heading order

and there is also the sectioning elements like aside which's outline shouldn't affect the page's outline.

It is pretty confusing, so at least I want to know how does it make better to have just one per page.