From the U.S. Census:
The U.S. Census Bureau launched a new online map today ahead of the 2020 Census apportionment results release. The “Historical Apportionment Data Map” currently displays apportionment results for each census from 1910 to 2010. 2020 Census apportionment results will be added to the map as they become available.
Apportionment is the process of dividing the 435 memberships, or seats, in the U.S. House of Representatives among the 50 states based on the apportionment population counts from the decennial census. Article I, Section 2 of the U.S. Constitution requires that a census of the population be taken every 10 years to apportion seats in Congress.