Early French estimates of the Illini population vary considerably because the different bands were constantly moving in and out of the large villages. Father Gabriel Dreuillettes in 1658 (written in Montreal) listed 20,000 Illini with 60 villages, but a few years later, Father Dablon at Sault Ste. Marie gave them only 2,000 and five villages. Marquette (1674) and Hennepin (1682), who actually visited them, both said there were 9,000, but neither included the Michigamea and Chepoussa bands in Arkansas. The best answer seems to be somewhere around 12,000. However, few would disagree that their population loss afterwards was dramatic. By the conclusion of the Beaver Wars in 1701, only 6,000 Illini and five of the original tribes remained. Epidemic and war continued their terrible toll, and the French in 1736 counted only 2,500. After neighboring tribes nearly destroyed them in 1769, the Illini were less than 1,800, only of whom 600 survived. Their number continued to fall: 480 in 1778; 250 in 1800; and 84 in 1854 when the remnants merged with the Wea and Piankashaw to become the United Peoria, Kaskaskia, Wea, and Piankashaw. The 1910 census listed the combined tribe at 128, but by 1937 the Peoria had grown to 370. Current enrollment is nearly 2,000.
http://www.tolatsga.org/ill.html