how to display an automatic number in a series as in an invoice number?
The simplest way is of course to number the first one (A1) 1, the one below =A1+1 and just copy the whole thing downwards - if you have the entries in one line.
Of course, this does not account for you having to erase some.
But I seem to recall that there is an auto-numbering feature somewhere?
An easier way is to type in the cell of your first invoice number the number you want to start with. Thereafter keep that cell selected and put the cursor on the small square at the bottom right corner of the cell. Press the left mouse button and pull down. The number increases automatically by one in each cell.
(if needed you can do the same pulling to the right side.)
This works only for numbers not for alphabetical characters.
That is the concept I was going for, too - it may be a better solution for when you want to delete a line.