To quote directly from Guelzo in
Gettysburg: The Last Invasion -
Based on the pay and muster reports recorded on June 30th, Meade should have had an army of approximately 112,000 men on hand, either for Pipe Creek or for Gettysburg. Determining the manpower of Civil War armies is a tricky business, compounded by lost or unsubmitted reports and diering denitions of what counted as “present” (which usually meant everyone who was issued rations) or “present for duty” (subtracting the sick but not the noncombatants) or “present for duty equipped” (those actually armed for the line of battle). In the 69th Pennsylvania, for example, the present and accounted for tallies on May 30th listed 389 men; but 52 of these were actually absent in hospital. Other men leaked away through desertion, and by the time they reached Gettysburg, the 69th could only count 292 on hand. In the 18th Massachusetts, the present-for-duty report listed 314 men, but the sergeant who “kept the company accounts” knew that only 108 “were found at the front” at Gettysburg. Meade himself believed that he had “about 95,000 … including all arms of service,” but in terms of troops ready to engage in combat, the Army of the Potomac was probably ready to furnish somewhere between 83,000 and 85,000 men.
The army’s real strength may have been more fragile even than that, since the expiration of many two-year enlistments from 1861 and emergency nine-monthers from 1862 had reduced the Army of the Potomac, after Chancellorsville, to as few as 40,000, and it was only by drawing some 37,000 troops from Schenck’s and Heintzelman’s garrisons in Baltimore and Washington that Meade was able to pull together a force worth challenging Lee. Units like George Stannard’s Vermont brigade, George Willard’s New York brigade (newly exchanged after being captured at Harpers Ferry in 1862 and cruelly mocked as the “Harper’s Ferry Cowards”), and Samuel Wylie Crawford’s Pennsylvania Reserve Division all increased the raw numbers of the army, but it remained to be seen how well they would t with the rest of the army, or even if they would fight at all.
To quote another section:
In the wake of McClellan’s dismissal in November 1862, and the debacle at Fredericksburg in December, desertions from the Army of the Potomac reached hemophiliac proportions—200 a day by one estimate, and over 25,000 by the end of January. Joe Hooker had, in a surprisingly skillful display of both carrot and stick, managed to restore and rebuild the army through the spring of 1863, bringing the number of deserters down to only 2,000 and ensuring that the army was well fed and well equipped. But the promises of victory that Hooker so lavishly spread around came to nothing at Chancellorsville in May; and what was worse, approximately 30,000 of the men in the ranks of the Army of the Potomac had been enlisted for two-year terms, rather than the customary three year volunteer service, or else had signed up for nine months’ service during the panic which greeted Lee’s invasion of Maryland in the fall of 1862. Those enlistments were due to expire in May and June 1863, taking away what seemed to one Minnesota soldier to be “fully one-half of the fighting strength of the old Army of the Potomac.”
This was the rankers’ view; in terms of actual numbers, the army could still field between 85,000 and 94,000 men (allowing for sickness, leaves, and assignment to rear-echelon duties). More serious damage was done to the inner workings of the army, which lost units and officers that had otherwise been part of the day-to-day machinery of divisions and corps. John Reynolds’ 1st Corps was reduced from 16,000 to 9,000 men, and within the 1st Corps, James Wadsworth’s division was shrunk from four to two brigades and John Cleveland Robinson’s division from three to two. Dan Sickles’ 3rd Corps was downsized from three divisions to two. Not even the commanders stayed still: not a single one of the major generals who would command a corps at Gettysburg had been in command ten and a half months before at Antietam; sixteen of the Army of the Potomac’s nineteen divisions got new commanding ocers between Antietam and Gettysburg
The Gettysburg Staff Ride also lists the May 31 strength of the Army of the Potomac at 95,000.