Numbering is consecutive but not sequential. e.g.
SECTION 10.213.3 PROTECTION
A. Protection of Vertical Opening
: (this means leaving or omitting other items)
F. Hazardous Area
G. etc.
“Numbering” in LibreOffice is something automatic. It exists to help people to create something monotonically increasing, so people don’t do repeated things like increasing numbers by one.
In your case, the task is necessarily manual (no way for the program to know what will you want on the next line). Why use numbering feature for that? It’s just as easy to type that manually… Or maybe using numbering ranges (i.e., fields which formulas may be freely edited manually where needed)?