##Minkolang 0.14, 18 bytes

$o7$ZsrlZ'26'$ZN.

Try it here.

###Explanation

$o                    Read in whole input as characters
  7$Z                 Uppercase every letter
     s                Sort
      r               Reverse
       lZ             Alphabet - uppercase and lowercase
         '26'         Pushes 26 on the stack
             0$Z      Count how often the top 26 numbers of the stack appear in the stack
                N.    Output as number and stop.

Minkolang 0.14, 18 bytes

$o7$ZsrlZ'26'$ZN.

Try it here.

Explanation

$o                    Read in whole input as characters
  7$Z                 Uppercase every letter
     s                Sort
      r               Reverse
       lZ             Alphabet - uppercase and lowercase
         '26'         Pushes 26 on the stack
             0$Z      Count how often the top 26 numbers of the stack appear in the stack
                N.    Output as number and stop.