`#`.

`#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):
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#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