|The Formula you want is:|
You could simply drag this down as far as you want and the 1's will increment 1 row at a time as you drag it down.
If you don't want the formulae just hanging around doing nothing until you add data, you could use a VBA WorksheetChange macro that would place the formula in the cell after you add data to the bottom of column E.
If you are interested in that option, let us know. We'd need to know a few more details, such as where the formula needs to go, etc.