The U.S. House of Representatives contains 435 members. This number is fixed at 435 and does not change as the country gains population — nor would it change if, for some reason, the United States were to experience a population decline.
Each individual state is awarded members of Congress based on its population, or the total number of people residing there. California is currently the most populous state, therefore it has 53 representatives in the U.S. House; while states with the lowest population each have only one representative in the House. (Alaska, Delaware, North Dakota, South Dakota, Vermont, and Wyoming are currently the only states that have a single representative in the House of Representatives.)
Every ten years, the United States government conducts a census, which documents current population trends across the country. This census is then used to revisit and re-factor how many representatives to the House each state will have. As the population of some states grows, they will gain representatives, while other states will lose representatives to keep the total number of representatives at 435.
The United States Senate is the other branch of Congress, and in the Senate, each state has two representatives (or Senators), regardless of population differences.