How to Insert a Page Break in a Numbered List in InDesign?

I would recommend to avoid manual page breaks. Why do you need them?

All desired break options can be set up via paragraph styles. This would help you to use any workaround here.

To add to what Scott has said, the last paragraph you want on the previous page ends with a paragraph return. Let's say we want the second page to start with number 7.

If you place the cursor at the end of paragraph 6 and insert a page break, the paragraph return will go along to the next page, and that's what gives you the extra blank number 7 on the second page.

If you place the cursor at the beginning of paragraph 7 and insert the page break, the paragraph return at the end of paragraph 6 will give you the blank number 7 on the first page.

In either case, you are using two paragraph breaks (a regular one, and a page break), and that's why you are getting an extra numbered line. Instead of just placing the cursor at the end of paragraph 6, double-click to select the paragraph return itself…

and replace it with the page break.

That way, you only have one type of break between those two paragraphs, and no extra numbered line. Showing the hidden characters (bottom of the Type menu) will help you be sure you have selected the paragraph return.

Because that page break is a paragraph, and that paragraph has numbering applied. Add the break at the end of the last paragraph you want on that page.