The U.S. Senate has 100 members, two from each of the countries 50 states. Article One of the United States Constitution specifies that each state shall have two Senators. This was part of the compromise that moved the Constitutional Convention past a potential deadlock on representation. The compromise also created the rules for the number of members that each state would have in the other house of Congress, the House of Representatives, saying that those seat numbers would be apportioned based on a state's population.

Members of the Senate were, until 1913, appointed by state legislatures. The passage of the 17th Amendment changed that, to afford election of members of the Senate by direct ballot, as with members of the House.

Senators serve six-year terms. The Senate has devised its membership so that every two years, when every member of the House of Representatives is up for re-election, one-third of the Senate is up for re-election. This was another part of the Constitutional Convention compromise.

Federal law does not prohibit an increase in the numbers of members of the U.S. Senate. Presumably, if the United States adds a state, then the Senate membership would increase to 102.

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