WOW! The answers are all over the board. I'll submit the first as my answer:
"As many as 500, some slave and some free, served in support roles for the Army of Virginia," per a writer for History.com.
Here is more definitive reasoning:
"There were no black combatants on either side at Gettysburg, only "noncombatants" in support roles: ambulance and supply-wagon drivers, hospital attendants, teamsters. Of those there were hundreds, Heiser explained, including, on the Southern side, personal body servants (i.e., slaves) tending to white officers. Paradis shows the same, arguing that black teamsters in particular faced hard, perilous conditions and at Gettysburg were vital to supplying the Army of the Potomac and helping the Army of Northern Virginia escape...It is impossible to identify the exact number of ex-slave and free blacks taken during the Gettysburg Campaign range anywhere from 30 to 40 to several hundred. Estimates are according to various first-person accounts. "
-- per Henry Louis Gates, Jr., as reported by John Heiser, Gettysburg National Military Park historian
And here is another source estimating support, but not specific to the Army of Northern Virginia:
"Nearly 180,000 Black Southerners, from Virginia alone, provided logistical support for the Confederate military. Many were highly skilled workers. These included a wide range of jobs: nurses, military engineers, teamsters, ordnance department workers, brakemen, firemen, harness makers, blacksmiths, wagonmakers, boatmen, mechanics, wheelwrights, etc." -- Sons of Confederate Veterans