lol
google up what 1U is if you dont understand what im about to explain

stackable server heights are standarized into "U"s, some think of it as amount of CPUs that can be put in it but rather its the height that makes the difference and even 1U can fit a dual board and such dual CPU boards do exist in 1U form

1U, 2U, 3U (rare), 4U
delta electronics makes the most 1U PSUs