I can't find my notes from Brandy Station but here is what I have from my very limited resources here in Texas:
The engagement began with the battery from Jenkins Cavalry Brigade Jackson's Va Battery of two 12lb Howitzers, and possible one other (not further identified) section (I suspect two guns from Griffin's 2nd Baltimore Battery as they were in the order of march in the advance at the time) this battery was two sections of 10lb Parrotts, being deployed on Cress ridge in an attempt to flush out the union cavalry forces sometime after 1pm.
Jackson's battery was driven back and replaced by the section of Green's Battery, Artillery Bn from Early's division attached to General Hampton the day before.
" On the 2d, I ordered two guns of Captain Green's (Louisiana Guard Artillery) battery, at the request of General Stuart, to report to General Hampton at Hunterstown, where he engaged the enemy, with a loss of I man killed and 1 sergeant and 14 men wounded."
From the OR report of HP Jones Bn Commander of Early's Div. This battery consisted of 1 section of 3" Rifles, and one section of 10lb Parrotts. It is not stated which guns were sent.
And at least one section from McGregor's Va (2nd Stuart Artilleriy of one section of 3" rifles and one 12lb Napoleon (I suspect it was two 12lb Napoleons but can not provide the reference) also deployed at this time. My references give six guns in position at this time all attributed to Green and McGregor's Batteries.
Brethead's Va Battery (1st Stuart Horse Artillery, two sections, both of 3" rifles) deployed after that in support on Cress ridge giving the Confederates a total of 10 guns in support at the time of the major action on the field. Most confederate and union fire was engaged in Counter-battery fire during this stage of the fight.
Confused yet? As a best educated guess without further references here is the entire artillery order of battle given for Stuart at the time:
Stuarts Horse Artillery (15 guns)
Breathed's Va (1st Stuart Artillery 4- 3" rifles)
Chew's Ashby Va battery with Jones' Bbe (2- 3" rifles, 2- 12lb Napoleons, 1- 6lb Howitzer) NOT at Gettysburg.
Griffin's 2nd Baltimore Maryland Battery 4-10lb Parrotts
Hart's SC Washington Battery 2-3" Rifles, 2-12lb Napoleons (originally Blackely Rifles but can not confirm they were still in service at the time)
McGregor's Va Lynchburg Artillery of Beaurgard Rifles (2nd Stuart Artillery 2-3" Rifles, 1-12 lb Napoleons
Moorman's Va Battery (4 guns not at Gettysburg) of 1-3 3" Rifles
Attached to Hampton's Bde
Green's Louisiana Guards Battery (one section attached) (2-3" rifles, or 2-10lb Parrotts) unidentified as to which section at this time.
Assigned to Jenkin's Bde
Cpt Thomas E. Jacksons Va Battery of one section of 2-12lb Howitzers
And here are the units engaged on East Battlefield during the brief engagement:
Assigned to Jenkin's Bde
Cpt Thomas E. Jacksons Va Battery of one section of 2-12lb Howitzers
Attached to Hampton's Bde
Green's Louisiana Guards Battery (one section attached) (2-3" rifles, or 2-10lb Parrotts) unidentified as to which section at this time
McGregor's Va Lynchburg Artillery of Beaurgard Rifles (2nd Stuart Artillery 2-3" Rifles, 1-12 lb Napoleon)
Breathed's Va (1st Stuart Artillery 4- 3" rifles)
Although I can not point to it I believe this battery might have been available towards the end of the engagement but was not deployed:
Griffin's 2nd Baltimore Maryland Battery 4-10lb Parrotts
So in total what I have is 10-11 guns broken down as follows:
one section of 2-12lb Howitzers
one section of 2-3" rifles, OR 2-10lb Parrotts
one battery of 2-3" Rifles, 1-12 lb Napoleons
one battery of two section of 2-3" Rifles
And in reserve but not engaged:
One battery of two section of 2-10lb Parrotts each (Griffin's)
VR
James Mattes