I believe that a good interface, taking into account clicks and clarity for all levels of user, would be as set out below. Since Lack has raised Advance in addition to the original Auto-Attack discussion I've included that too.
I think that auto-attacking, to a user-defined army limit; "whilst I've got 20 armies or more left" for example, is better than for a user-defined number of attacks. (Note "whilst I've got 4 armies or more left" is the equivalent of the current auto-attack.) Limiting by number of attacks still leaves you particularly vulnerable to:
joeyjordison wrote:... got down to u havin 2 and them havin 6 ...
... if I understand his meaning correctly.
Note that, in my personal-just-made-it-up-notation, this:
 is a dropdown
<Attack> is a button.
<Auto-Attack> with  armies or more.
The dropdown list defaults to 4, since that is the current auto attack equivalent, but it will be coded to include all values from 2 up to one less than the maximum number of armies the player has on any one territory. (There will have to be some checking producing a warning similar to the existing "illegal attack" warning if a dropdown value inappropriate to the attacking territory is chosen.)
This interface allows players to be more cautious or more reckless than the current auto-attack allows and with the efficiency of the auto-attack button.
<Advance All> armies from East Anglia to Lincolnshire or
<Advance Specified> 
The dropdown list defaults to zero since this allows an "advance none" order with one click and doesn't add too much clutter to the interface. (The dropdown, as now, will be coded to included all values up to one less than the number of armies in the 'from' territory.)
The italicised 'or' is intended simply as that - italicised text on the page.