Calculates the number of variables in a given data set.
- is the input variables data set (two dimensional array of cells, where each column corresponds to a variable).
- is the boolean array to choose the input variables in the set. If missing, all variables in X are included.
- In case the number of the size of the mask array is smaller than the number of variables in X, the array is appended with ones to match X.
- In case the number of the size of the mask array is greater than the number of variables in X, the excess elements are dropped/ignored.