What term is used to describe the political party that has the most members in each house of Congress

It is called the "majority" for practical and formal purposes.

Explanation:

After a legislative election, the party that has won the largest number of seats in each house of the American Congress is simply called "the majority," and the other party is called the "minority." For example, the leader of the caucus that has most representatives in the House of Representatives is called the "majority leader," and this person comes second to the Speaker of the House, who usually is an influential figure of the majority party.