Section 2 of the 14th Amendment mandates that the apportionment of Representatives among the states be based on "counting the whole number of persons in each State, excluding Indians not taxed". This means that the census counts everyone residing in a state for the purpose of determining that state's representation in Congress.
Article I, Section 2 of the U.S. Constitution mandates that an apportionment of representatives among the states must be carried out every 10 years