Still wondering about the answer to this one ...?
I still have many questions about this, myself. Here's where my thinking is, at this point:
The manual states that frontage can vary between 2 and 8 elements.
Pocus has said that the notion of a maximum of 4 arty in any frontage is a myth.
The clearer the terrain, the more artillery elements can be in frontage. Conversely, with more wooded the terrain, the fewer artillery elements.
From the AACW wiki:
[INDENT]The frontage space occupied by a sub-unit depends on its maneuverability in this type of terrain.
Example: On the one hand, regular units are quite slow in mountain terrain and take up much frontage space in such terrain as a consequence. Partisans, on the other hand, are much faster in mountains and take up less frontage space. This means you could engage the enemy with many more partisans than regular units in this particular case. [/INDENT]
Elsewhere, I've proposed the idea of Divisional units defined by their ordnance. Here's a condensed summary.
[INDENT][SIZE="1"]...For example, consider a Mountain Division. Within wilderness, frontage is going to be poor, and ranges are going to be shorter. Consequently, it is unlikely that the commonly accepted value of 4 ordnance per division will be of much use, with such limited frontage. The paired battery of 20 lb Parrott and 12 lbr mentioned earlier would serve well here. Meanwhile, the two extra element slots within the Division might be used for Cav, instead.
Another example might be a Striker Division, defined as a division which would generally work independently of a corps or army. Such a division might include 5 artillery units: a Columbiad, a 20 lb Parrot, two 10 lb Parrots, and a 12 lbr. I would further recommend the inclusion of more Cav than the usual 1; at least two, and possibly as many as four. I also believe that this division's infantry should consist entirely of what I call "Rifle" Infantry: Infantry with a range of 4. Any availible Elite infantry (with its cohesion bonus) should be placed in striker divisions, first.
Yet another example might be a Cavalry Division, with ordnance consisting of 2 Columbiads, one12/20 pair, and a pair of HA, for a total of 6 artillery. 6 Cav and 5 Infantry would fill out the rest. Of course, this Cav division would be best suited for operations in open terrain, with maximum frontage. (Note that the HA pair can be detached with one or more Cav elements for excursions to recon or raid.) If any division is in need of an Executive Officer (XO), a Cav Division is. The XO can accompany excursion forces.[/size][/INDENT]
Here's a link to the discussion I mentioned. The attachments explain my concept of WEI and so on.