In the United States Congress quorum is made up of the simple majority of representatives. This mean quorum in the Senate is 51 and in the House of Representatives is 218.
Yes, they are. They are on the committee by way of their office, but they still have the same rights as other members, and still count for purposes of a quorom.
Congress has to meet once a year because Section 4 of Article 1 in the United States Constitution requires it. This is because Congress needs to examine the state of the country, determine if legislation is needed, and, as stated in Section 5, " be the judge of the elections, returns and qualifications of its own members, and a majority of each shall constitute a quorom to do business." A "quorom" is the number of members required to be present for business to be legally conducted.