19. the answer is...

six (6)

The Framers of the Constitution wanted senators to be independent from public opinion. They thought a fairly long, six-year term would give them this perfection. They also wanted longer Senate terms to balance the shorter two-year terms of the members of the House, who would more closely follow public opinion. The Constitution puts no limit on the number of terms a senator may serve. Elections of U.S. senators take place on even-numbered years. Every two years, one-third of the senators are up for election.