I had trouble finding online source that gives numbers. My searches about African-Americans/blacks attached to the ANV at G'burg come up with varying numbers of free Pennsylvania African-Americans seized and sent back to the South to be sold into slavery, but they do not appear to be included in the question, since they weren't used to help the ANV during the Gettysburg campaign.
One quote I found: "Thousands of slaves served in support roles for the Army of Northern Virginia, and as Lee's army marched north into Pennsylvania, they seized as many as 500 African-Americans—some former slaves, some free their entire lives—and brought them back to Virginia to be sold into slavery." So those 500 were not working for the ANV during the invasion, and theres no mention of how many of the "thousands" went north with the ANV.
https://www.history.com/news/7-things-you-should-know-about-the-battle-of-gettysburg
I suspected that this book by Eed Bearss and James Paradis might have the answer:
https://www.worldcat.org/title/african-americans-and-the-gettysburg-campaign/oclc/56214106, but it is a recent book (2005) so is not available online. However, the amazon.com description says it's about Gettysburg citizens, who of course were not "toiling" for the ANV. This review of the book
https://www.pennlive.com/opinion/2013/06/gettysburg_african-americans_role_union_confederacy.html mentions "black Confederates" serving the ANV as teamsters during the invasion, but mentions no numbers.
I don't know if this is the source to which the question author referred, but I finally found this post on CWT:
https://civilwartalk.com/threads/li...th-the-confederate-army-at-gettysburg.149419/ This is probably as good as we are going to get:
"As of August 2018, from primary sources collected or reviewed over the past 30 years, I have confirmed the presence of 72 slaves who accompanied the Confederate Army of Northern Virginia during the Gettysburg campaign. I also show four who were probably present, for a grand total of 76. Four others who were present were identified as free-born. All are listed below, with the name of their owner or unit to which they were attached, duties (if described), and fate (if known). I have not included the names of 33 others who were possibly present.
"I estimate slaves and freemen together comprised 4.6 percent of the Confederate army's strength at Gettysburg, although they are not included in the tally of 80,025 in the army (figure from Busey and Busey, Regimental Strengths and Losses at Gettysburg).
It would mean that between 3,500 and 4,000 blacks were brought to the vicinity of Gettysburg with the Confederate army, with about 95 percent of them being slaves." [emphasis mine]
So we have 72 or 76 or 80 or 113 confirmed. Two other posters on that thread added 6 more, so that makes a possible 119 confirmed. However, the original poster in his second paragraph estimates a total "
between 3,500 and 4,000," so I'll go with that.