21. In $\frac{1 \times 2}{2}, \frac{1 \times 2 \times 3}{2^{2}}, \frac{1 \times 2 \times 3 \times 4}{2^{3}}, \cdots \cdots, \frac{1 \times 2 \times 3 \times \cdots \times 100}{2^{99}}$, the number of terms that simplify to integers is $\qquad$ .
$6$