Keys-space for a restricted substitution cipher
It is called derangement, see Wikipedia.
Also, in Mathematics Stack Exchange 1, 2 

Number of derangements of n elements $=\sum\limits_{k = 0}^{n}(-1)^k\frac{n!}{k!}$

When n = 26 the result is 148362637348470135821287825 $\approx 2^{87} $