28

Number
$28$ (twenty-eight) is:


 * $2^2 \times 7$


 * The $7$th triangular number after $1, 3, 6, 10, 15, 21$:
 * $28 = 1 + 2 + 3 + 4 + 5 + 6 + 7 = \dfrac {7 \times \left({7 + 1}\right)} 2$
 * Hence there are $28$ dominoes in a standard set.