Enter the formula =ROW() in the first cell and copy it down. (You might
like to add or subtract a constant depending on how you want the cells
numbered). When you delete a row this will automatically re-number.

If you insert a row, though, you will still need to copy the formula
down, so this only partially meets your requirements. There again,
though, you might have other formulae to copy into the new row, so you
might do this at the same time.

Hope this helps

Pete