To answer the above question, the relevant sentence is: Elections to the United States Senate will be held November 6, 2018, with 33 of the 100 seats in the Senate being contested in regular elections.
This justifies what answer for what question? Q & A:
Passage: Elections to the United States Senate will be held November 6, 2018, with 33 of the 100 seats in the Senate being contested in regular elections. The winners will serve six-year terms from January 3, 2019, to January 3, 2025. Currently, Democrats have 23 seats up for election along with 2 independents who caucus with them. Republicans have 8 seats up for election. Two of the Republican seats are open as a result of Tennessee Senator Bob Corker's and Arizona Senator Jeff Flake's pending retirements. The seats up for election in 2018 were last up for election in 2012, although some seats may have special elections if incumbents die or resign, as has already happened in Alabama and Minnesota. After the 2016 elections, some state election officials are trying to upgrade voting systems in time for this election.

Question: Based on this passage, how many senate seats are up for election in 2018?
33