There are 26! or 4 * 10^26 possible keys available for the monoalphabetic substitution cipher. 

Explanation:

For monoalphabetic substitution cipher, we use a permutation of alphabets.
a permutation is explained as

Let's say S is an ordered sequence of elements, with each element appearing exactly once. 
For example, if S = {x, y, z}, there are six permutations of S:
 (xyz, yxz, zyx, xzy, yzx, zxy)

in general, there are n! permutations for a set of n elements.
in this case, there are 3 elements. i.e., 3! permutations which equal 6.

In Caeser cipher, if take key is 3, 
plain:      a  b  c  d  e  f  g  h   i  j  k  l  m  n  o   p  q  r  s   t   u   v  w   x  y   z
cipher:    D E  F G  H  I  J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z  A  B  C

If, instead of using cipher line and leaving the attacker a good chance of cracking the key(only 25 tries), we can replace the cipher line with one of the permutations of alphabets
 i.e., giving 26! or 4 * 10^26 possibilities.
giving one hell of a headache for the attacker to find the key. 

But, there is one weak point in this technique, if the cryptanalyst knows the language of plaintext, he can exploit the frequency of letters to find the key.

This approach is known as a monoalphabetic substitution cipher. substantially reducing the chances of finding the key through brute force analysis.