Thomas Sumter claimed that he had “Six hundred brave men,”  and other participant statements point to a similar total . Not everyone in his command was armed or fought during the battle (among these was Andrew Jackson, future 7th president of the United States) . The number of men who were engaged may have been closer to 540 . Those who were armed were badly supplied with ammunition. It was thought that Sumter's men had no more than 5  or 10  bullets apiece.
Sumter divided his force into three parts, and each contained around 200 men. These divisions are henceforth referred to as the left, center, and right divisions, after their relative positions at the beginning of the battle. Possibly the left division was commanded by William Hill, the center by Robert Irwin, and the right by Richard Winn .
Left Division: Several small South Carolina militia regiments, including William Bratton’s regiment, Andrew Neal’s regiment (commanded by either Samuel Watson or William Hill ), Edward Lacey’s regiment (possibly commanded by John McClure ), and Joseph Brandon’s regiment (commanded by James Steen) . Most of the men in these regiments had seen action before, including at Williamson’s Plantation and Rocky Mount. Of the men that fought in this division, Joseph McJunkin of Brandon’s regiment left a detailed description of the battle.
Center Division: A battalion of 200 North Carolina militiamen under the command of Robert Irwin . These men hailed from Mecklenburg County. Of the men that fought in this division, Joseph Graham left a detailed description of the battle.
Right Division: Major William Davie’s troop of dragoons (70-80 men) , supported by a mixture of North and South Carolina militia units . Of the men that fought in this division, William Davie and Richard Winn left detailed descriptions of the battle.
Thomas Sumter claimed that he lost 20 killed, 40 wounded, and 10 missing . Other American participants gave lower figures, but their claims are less reliable. The 10 missing men were likely killed or wounded, but as the fighting took place over a wide area, they were not found by their comrades before the end of the battle. Among these was Matthew McClurkin, who "was severely wounded on the head... taken prisoner by the British troops... confined in jail and... sent to Charleston to be put aboard a prison ship." Fortunately, before arriving in Charleston, "he and the other prisoners overpowered the guard and made their escape" 
The British estimated American losses at about 100 .
1. Letter from Thomas Sumter to Thomas Pinckney, August 9, 1780.
2. James Hodge Saye. Memoirs of Major Joseph McJunkin, Revolutionary Patriot; Alexander S. Salley. (1921). Col. William Hill's Memoirs of the Revolution.
3. Augustus C. Buell. (1907). History of Andrew Jackson: Pioneer, Patriot, Soldier, Politician, President.
4. According to Joseph McJunkin. Richard Winn claimed that Sumter had 500 men with arms and another 200 without. The men without arms were left to take care of the horses, but later these men took up the arms of the killed and wounded and joined their comrades in the action.
5. Hill, ibid.
6. William Gordon. (1801). The History of the Rise, Progress, and Establishment of the United States of America, Vol 3.
7. This description is based primarily on the account of William Davie. See The Revolutionary War Sketches of William R. Davie [extract]. Davie's account also appears in, John H. Wheeler. (1851). Historical Sketches of North Carolina from 1584 to 1851, Vol. 1. Davie identified Hill and Irwin with the left and center, respectively. Davie claimed he had command of the right division, but Richard Winn also claimed that role. Winn's claim seems more credible as he outranked Davie and had served with Sumter longer.
8. Andrew Neal was killed 7 days earlier at Rocky Mount. One Samuel Gordon of this regiment claimed that Samuel Watson took over after Neal was killed, whereas Thomas Lofton remembered Hill becoming his commander. See the pension application of Samuel Gordon, and the pension application of Thomas Lofton, both transcribed by Will Graves.
9. Joseph Gaston claimed that before Sumter's men crossed the Catawba River to attack Hanging Rock, "General Sumter ordered an election for general officers in the Chester Regiment." He claimed that John McClure was elected colonel, "Colonel E. Lacy having, at that time, become unpopular among the Chester Whigs." However, several accounts placed Lacey at the battle, suggesting either that Gaston was in error or that he was serving in some other capacity. See Joseph Gaston. (1836/1873). Joseph Gaston's Narrative. The Historical Magazine and Notes and Querries Concerning the Antiquities, History and Biography of America, Vol. 1.
10. Will Graves. (2005). What Did Joseph McJunkin Really Saye? Southern Campaigns of the American Revolution, Volume 2, Issue 11. Brandon's absence on this occasion is not explained in the available sources. Steen was his second in command.
11. William Davie, ibid. Letter from Allen J. Davie to Archibald D. Murphey, August 9, 1826.
12. Graham said 70; Hill, ibid., said 80. William Davie, ibid., placed his troop on the American right. For Graham, see William A. Graham (1904). General Joseph Graham and His Papers on North Carolina Revolutionary History.
13. William Davie, ibid. Winn, ibid., identified his regiment as one of the specific units.
14. Sumter, ibid.
15. The pension application of Matthew McClurkin, transcribed by Will Graves.
16. Banastre Tarleton. (1787). A History of the Campaigns of 1780 and 1781. Charles Stedman. (1794). The History of the Origin, Progress, and Termination of the American War, Vol. 2.