In the k programming language, count is the monadic case of #.

#x returns 1 if x is an atom. Otherwise, x is a vector, array, or list of some sort, and #x returns the number of elements on the first axis or top level of x.

Illustrated at the k command prompt (user input indented 2 spaces, results flush left):

  #1 2 3
3
  #1
1
  #_n
1
  #!0
0
  a:(1 2 3; 4 5 6; 7 8 9; 10 11 12)
  a
(1 2 3
 4 5 6
 7 8 9
 10 11 12)
  #a
4
  #:'a
3 3 3 3
  b:(1;1 2;1 2 3)
  b
(1
 1 2
 1 2 3)
  #b
3
  #:'b
1 2 3