How many rivers that have their origins in Canada, flow into the United States?

At least for the major rivers it looks like it's two - the Yukon and Columbia

see

https://en.wikipedia.org/wiki/List_of_longest_rivers_of_the_United_States_%28by_main_stem%29

'...Four-the Yukon, Columbia, Porcupine, and Kootenay-begin in Canada and flow into the United States. Five-the Milk, Pend d'Oreille, Saint Lawrence, Red, and Saint John-begin in the United States and flow into Canada. Of these, the Milk and the Kootenay cross the international border twice, the Milk leaving and then re-entering the United States, the Kootenay leaving and then re-entering Canada. The drainage basins of these nine rivers extend into both countries; in addition, the drainage basins of six others-the Fraser, Assiniboine, South Saskatchewan, Saskatchewan, Nelson, and Winnipeg-extend into the United States even though their main stems flow entirely within Canada.'

https://en.wikipedia.org/wiki/List_of_longest_rivers_of_Canada