An independent input serial number in each cell is a long process, much easier to use AutoFill. The fill handle (the frame with a small square in the lower right corner) is displayed in the program by default. If for some reason in your version it is not, turn it on.
To do this click on the Office button in the upper left corner of the window and select the menu "Excel Options". This will open a new dialog box. Navigate to it in the "Advanced" section. In the "edit Options" select the check box for "Enable fill handles and drag-and-drop cells". Apply the new settings by pressing the OK button in the lower right corner of the window.
Keep in the first cell of the first serial number in the next cell, the second sequence number. Select filled cells and move the mouse cursor to the small square in the right corner of the frame. Press the left mouse button and holding it drag the frame in the desired direction. Empty cells are automatically marked with the missing sequence numbers.
The option of using context menu, autocomplete: enter the first cell of the first sequence number, drag the frame in the desired direction for the desired number of cells. Release the left mouse button near the marker icon will appear in the minimized menu the "autocomplete Settings", click on it with the left mouse button and set the marker next to "Fill" in empty cells will be prescribed rooms.
For numbering you can use a simple formula. Enter in the first cell of the first sequence number, move the cursor to the second cell and type in the formula bar an equal sign. Click the left mouse button in the first cell, then enter without quotes "+1" and press Enter. Select the cell with the formula and drag its outline to the desired number of cells. In the end you'll have a formula for the second cell: =A1+1, for the third cell: =A2+1, for fourth: =A3+1.
If you use a multilevel list for example, 1.1, 1.2, 1.3 and so on, set the cells to text format so that the program does not change the sequence number to the name of the month. To do this, click on the cell (range of cells), right-click and select the menu item "Format cells". In the opened dialog box, select the desired format in the tab "Number".