k1 subitizes (effortlessly/immediately/instantly counts the number in/recognizes the cardinality of) set k2 to be k3 [number/count], counted by/in terms of units k4
"Counting off" does not really make sense for this word. Must be extremely rapid without very much active tabulation/numeric processing/calculation; also must be confident and accurate in order to be true subitization (although this word my apply to false/imperfect subitization). See also: tolsurla, kancu.