I’ve been interested in finding these numbers for a while, and so am putting them here so I’ll be able to find them again
From The Myth of the Rational Voter, page 8:
“About half of americans don’t know that each state has 2 senators, adn three-quarters do not know the length of their terms. About 70% can say which party controls the House, and 60% the senate. Over half cannot name their congressman, and 40% can not name either of their senators. Slightly lower percentages know their representatives’ party affiliations”.