The answer is more complex than you might think! Most people might say 535, but there are actually 2 correct answers: 541 and 542. To briefly explain each one:
Most people would count the total number of Senators (100) and the total number of voting Representatives (435) to come up with 535.
However, the question was not "how many voting Members of Congress are there?". There are 6 Members of Congress who cannot vote, but who nonetheless are Members with the right to sit on committees, speak on the Floor of the House, and otherwise conduct the business of Congress, with the exception of voting. These 6 Members are the delegates from the District of Columbia, the US Virgin Islands, the Northern Mariana Islands, Guam, and American Samoa, plus the Resident Commissioner of Puerto Rico.
Lastly, the Vice President of the United States is explicitly made President of the Senate by the Constitution, has the right to preside over the Senate, and breaks ties in Senate floor votes. For this reason, he is occasionally counted as a "member" of the Senate, but most scholars exclude him from the definition of "Member of Congress" because his position is clearly within the executive branch, and because he cannot take part in any other formal Senate business. Nevertheless, I think you could fairly say there are 542 total "Members of Congress" if you count the Vice President.