Final answer:
The U.S. Senate has 100 seats, with each state contributing two Senators who are elected to serve six-year terms. This equal representation is mandated by the U.S. Constitution and was further defined by the Seventeenth Amendment, which allows for direct election of Senators by state voters.
Step-by-step explanation:
The number of seats in the U.S. Senate is determined by the Constitution of the United States, which stipulates that each state is allotted exactly two Senators, regardless of its population size. These Senators serve six-year terms, and originally, they were chosen by their state legislatures as per Article I, Section 3 of the Constitution.
However, following the ratification of the Seventeenth Amendment in 1913, senators are now elected directly by the voters in each state. Therefore, with fifty states in the Union, there are a total of 100 seats in the U.S. Senate.
This is in contrast to the House of Representatives, where the number of seats is apportioned based on each state's population, as determined by the decennial U.S. Census, with a fixed total of 435 representatives.