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The seats in the Senate are fixed at two per state. In the House of representatives, the distribution of districts within the states are based on population, and may be adjusted based on the official US Census taken every 10 years.
Every ten years, due to the census. The House of Representatives just changes the number of people they represent, not the number of seats. It's set at 435 seats for the House, and 100 for the Senate...
The number of districts per state is apportioned based on population, and is determined (and adjusted as needed) by the official Census every 10 years.