Some comments regarding Confederate artillery ammunition at the close of the battle on July 3:
"The batteries are generally short of ammunition" (Gen. Robert E. Lee, in a conversation held with Brig. Gen. Imboden around 1 a.m. on July 4)
"Our army was held there [near Hagerstown on the retreat] with little or no ammunition." (Channing Bolton, Engineering Officer, Pender's staff)
"The officer [in charge of the reserve ordnance train of the army] directed me to inform Gen. Pendleton that the supply in the reserve train was getting low [as of late afternoon on July 3], and if the expenditure which was then being made continued much longer, we would not have enough to supply the demand ... Some of that ammunition marked and inspected and certified to by the Confederate States Ordnance Department at Richmond, Virginia was opened; I saw it with my own eyes; and yet it would not fit a single gun in the artillery of the Army of Northern Virginia, and was intended for siege guns." (Reminiscences of Coupland R. Page, on Brig. Gen. Pendleton's staff)
"July 4 … I was near a conference of high ranking artillery officers and heard [Col.] E. P. Alexander say that we did not have left more than four rounds of ammunition to each gun." (Capt. William W. Chamberlaine, Memoirs of the Civil War - on Col. Walker's staff of the Third Corps artillery)
That being said, the short-range 12-pounder Howitzers in the army had nearly all been idle during the battle and presumably had full, or nearly full, chests. Here are the numbers of those howitzers available in the various battalions:
Cabell - 2
Eshleman - 2
Alexander - 4
Poague - 6
Lane - 5
Pegram - 2
Garnett - 2
Total - 23
Those 23 Howitzers would have been gainfully employed against a Federal counterattack against Lee's center. No doubt they would done considerable damage against, for example, a massed Sixth Corps assault, but they were not sufficient in numbers to inflict what in my opinion would be considered "devastating" damage, that is, to turn back an all-out Federal effort given the huge gaps left in the center of Lee's army after the repulse of Pickett, Pettigrew and Trimble's brigades.