how is the number of electors per state determined?

The number of electors per state in the U.S. Electoral College is determined by the total number of that state's congressional delegation. This means each state gets a number of electors equal to the sum of its two senators plus its number of representatives in the House, which is based on the state's population as determined by the decennial U.S. Census. Every state has two senators, and at least one representative, so the minimum number of electors per state is three. The Census population determines how many House seats (and thus electors) each state receives, using a method called the Method of Equal Proportions for apportionment. Washington, D.C. is allocated three electors per the 23rd Amendment, equivalent to the least populous state, even though it is not a state. Overall, there are 538 electors: 435 for the House, 100 for the Senate, and 3 for D.C. The apportionment changes every ten years with Census updates, affecting the number of electors states have in presidential elections starting with the one after the Census.