Reapportionment
The number of seats in the House of Representatives is currently set to 435, and has been since 1913, except for a temporary increase to 437 after the admissions of Alaska and Hawaii. Though the actual reapportionment will normally occur in respect of a decennial census, the law that governs the total number of representatives and the method of apportionment to be carried into force at that time can be created prior to the census.
The decennial apportionment also determines the size of each state's representation in the U.S. Electoral College—that is, any state's number of electors equals the size of its total congressional delegation (i.e., House seat(s) plus Senate seats).
Federal law requires the Clerk of the House of Representatives to notify each state government of its entitled number of seats no later than January 25 of the year immediately following the census. After seats have been reapportioned, each state determines the boundaries of congressional districts—geographical areas within the state of approximately equal population—in a process called redistricting. Any citizen of the State can challenge the constitutionality of the redistricting in their US district court.
Because the deadline for the House Clerk to report the results does not occur until the following January, and the states need sufficient time to perform the redistricting, the decennial census does not affect the elections that are held during that same year. For example, the electoral college apportionment during 2000 presidential election was still based on the 1990 census results. Likewise, the congressional districts and the electoral college during the 2020 general elections will still be based on the 2010 census.
Read more about this topic: United States Congressional Apportionment