The number of seats in the House of Representatives is determined by population statistics based on the results of the decennial census conducted by the United States Census Bureau. The process is mandated by the Constitution and called apportionment. Congressional districts are redrawn every ten years so that an equal amount of the population is represented in Congress. “Redistricting is the process of revising the geographic boundaries of areas from which people elect representatives to the U.S. House of Representatives, a state legislature, a county or city council, a school board, etc.”
Redistricting is sometimes complicated by the practice of "gerrymandering," which is very simply defined in the Dictionary of Americanisms (1859) as “arranging the political divisions of a State so that, in an election, one party may obtain an advantage over its opponent, even though the latter may possess a majority of votes in the State."