How do you fit 80+ servers in one rack?

Question

Server equipment

Hello! Continuing to delve into the intricacies of the infrastructure, I came across wordings like "One 42U rack can accommodate up to 84 servers" and did not understand how)) I thought that 1U = 1 server, I suspect that the number of processors in 1U is 2 processors. I understand correctly ? ))

Answer 1

https://ru.m.wikipedia.org/wiki/Blade server

Answer 2

and it's not even a fifth of the rack! =)