insert a column and number the rows like that instead of depending on row numbers. You can't change row numbers themselves
When you insert a row it will renumber your rows below the new row automatically. You will need to click on the row number not the cell. The new row will appear above the row number you selected.
If you physically typed the number labels in Column A instead of using the Column numbers in the gray you could use the number series from the last point that your at. (Example, since you want a row in between 12,13 you would insert the row and do your number series starting with 11, 12. Other than that I'm not sure.