A family of Microsoft word processing software products for creating web, email, and print documents.
You're running up against a conflict with Word's automatic list numbering feature. You could turn of the Automatic Numbered Lists feature in Tools> AutoCorrect> AutoFormat As You Type but for what you're describing it would probably be better to use a 2-column Table of however many rows you prefer. Put the 'line numbers' in the first column & format the table to have bottom borders on the right column only. If you aren't familiar with the use of Tables go into Word Help, search using the keywords table tips then watch the video entitled QuickTip - Word table tips.
Regards,
Bob J.