This is the concept of median of statistical data, given that Mary was the 13th highest and the 13th lowest in the spelling contest it implies that the number of people was distributed as follows; 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25 This implies that since the median of 25 is 13, then the total number of people in the contest was 25
Residual standard deviation is the square root of the residual variance. It is an estimate of the error made on the measurement of the dependent variable.
Thus, the spread around the regression model is usually measured with the residual standard deviation because it is the measurement of standard error between actual and predicted value.
