Sometimes 2 are used
A typical txp uses only 1 because the function of that organ suffices to support life.
The living donor can survive with 1 remaining kidney. A deceased donor's kidneys can be used to benefit 2 different pts - crucial with the severe organ shortage. But 2 small pediatric kidneys may kept en bloc permitting use of large vessels, or 2 adult kidneys with low GFR are strategies to expand the donor pool.