Ballotpedia – The encyclopedia of American politics: “A total of 468 seats in the U.S. Congress (33 Senate seats and all 435 House seats) are up for election on November 3, 2020.

• U.S. Senate – There are 11 Democratic seats, 20 Republican seats, and two seats currently waiting to be filled via special election on November 6, 2018, up for election in 2020. In 2014, the Republican Party picked up nine seats, resulting in their having more seats to defend in 2020. The map…shows what seats are up for election and the current incumbent in each race.
• U.S. House – All 435 U.S. House seats will be up for election….” [h/t Pete Weiss]