65 ILCS 5/5-2-2
Except as otherwise provided in Section 5-2-3, the number of alderpersons, when not elected by the minority representation plan, shall be as follows: In cities not exceeding 3,000 inhabitants, 6 alderpersons; exceeding 3,000, but not exceeding 15,000, 8 alderpersons; exceeding 15,000 but not exceeding 20,000, 10 alderpersons; exceeding 20,000 but not exceeding 30,000, 14 alderpersons; and 2 additional alderpersons for every 20,000 inhabitants over 30,000. In all cities of less than 500,000, 20 alderpersons shall be the maximum number permitted except as otherwise provided in the case of alderpersons-at-large. No redistricting shall be required in order to reduce the number of alderpersons heretofore provided for. Two alderpersons shall be elected to represent each ward.
If it appears from any census specified in Section 5-2-5 and taken not earlier than 1940 that any city has the requisite number of inhabitants to authorize it to increase the number of alderpersons, the city council shall immediately proceed to redistrict the city in accordance with the provisions of Section 5-2-5, and it shall hold the next city election in accordance with the new redistricting. At this election the alderpersons whose terms of office are not expiring shall be considered alderpersons for the new wards respectively in which their residences are situated. At this election a candidate for alderperson may be elected from any ward that contains a part of the ward in which he or she resided at least one year next preceding the election that follows the redistricting, and, if elected, that person may be reelected from the new ward he or she represents if he or she resides in that ward for at least one year next preceding reelection. If there are 2 or more alderpersons with terms of office not expiring and residing in the same ward under the new redistricting, the alderperson who holds over for that ward shall be determined by lot in the presence of the city council, in whatever manner the council shall direct and all other alderpersons shall fill their unexpired terms as alderpersons-at-large. The alderpersons-at-large, if any, shall have the same power and duties as all other alderpersons but upon expiration of their terms the offices of alderpersons-at-large shall be abolished.
If the redistricting results in one or more wards in which no alderpersons reside whose terms of office have not expired, 2 alderpersons shall be elected in accordance with the provisions of Section 5-2-8.
65 ILCS 5/5-2-2