12 is a standard number that can be broken down and scaled up quite well. Putting in an extra horse for officers, standard, musician or the odd guy out would not hurt, but as we likely pay for it in money or less goodies on the sprue…
More important to me would be the question of the POSE. I have galloping horses in all variations – looking perfectly dynamic and terrible if I place my army on the table. Trotting, walking or standing horses are rare, though they make up for 99.8% of the actual battlefield time of cavalry. If given the chance I would buy standing or walking horses if I could – though I am quite sure the majority of customers likes them in full charge. :-/
Even a unit in trotting would be a welcome change.
That said, standing horse can combine left/right side far better then actually moving horses, and you need at least three different poses to make any unit believable. Perhaps different sprues for horse & rider would be better, giving the option to reuse the horse for different unit types.
Another important aspect – to me – is horse size. There are companies out there who have a “one size fits all” policy, producing knights horses and steppe ponies of the same shoulder height. Please make sure that the horses will fit to their historical prototypes. Roman cavalry should not use the same horse as British Napoleonic Dragoons.