The term of office for a US Senator is six years. Approximately one third of the Senate is up for election during each even numbered election cycle.
six
U.S. Senate seats are divided among three classes. The classes have nothing to do with rank; they determine only the beginning and end dates for the three overlapping U.S. Senate terms. At the time statehood is granted, the two Senate seats to which each state is entitled are added to two of the classes. Class 1 Senate seats come up for election every six years including 2012. Class 2 Senate seats come up for election every six years including 2014. Class 3 Senate seats come up for election every six years including 2016.
two for each state
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In each two year election cycle one-third of the United States Senate is up for re-election as each Senator serves a six year term. In the United States in 2012 23 Democratic seats were up for election and 10 Republican seats. (seats either held by a Democrat or Republican) The Democrats won all of their seats back and turned two Republican seats into Democratic seats. Two other seats were won by Independents; one of them previously a Republican seat. There are now 55 Democratic seats and 45 Republican seats in the United States Senate as the two Independents are caucusing with the Democrats.
There are 100 Senate seats, two from each State
34 U. S. Senate seats, one from each of 34 states, come up for election every six years, including 2016. 33 U. S. Senate seats, one from each of 33 states, come up for election in every other even-numbered year.
Two seats
Every two years, all of 435 congressional seats are up for election. In the Senate, however, their terms are staggered and approximately one- third of 100 the seats are up or election every two years since. Senators do serve for six years.
There are always two U. S. Senate seats for each state, so since 1959 there have been 100 seats in the Senate.
Senate
Elections - every 3 years (or sooner, but not longer) the House of Representatives is dissolved and the people of Australia vote to elect their member of parliament. In the Senate a term is 6 years, so at each election half the senate seats are vacated and put up for election.
There are 535 seats in both houses, 100 seats in the Senate and 435 in the House of Representatives. The Senate has two members from each of the fifty states. The House has at least one member from each state, with the rest portioned out according to each state's population, based on the latest Census.