Which states have only one representative in the house of representatives?

There are currently seven states that only have one Representative: Alaska, Delaware, Montana, North Dakota, South Dakota, Vermont, and Wyoming.

There are other territories that don't have any Representatives in the House, but they do have a different kind of representation—the District of Columbia, Guam, the U.S. Virgin Islands, the Commonwealth of the Northern Mariana Islands, and American Samoa each have one non-voting Delegate, and Puerto Rico has a non-voting Resident Commissioner that fills much the same role in the House. They can't vote, but they can serve on committees.

Representatives are allocated by population, which is counted every ten years by the census. This year, 2020, is a census year, and the House will be automatically reapportioned based on that data in 2023.