I did the VOI (Volume of interest) on my image. But the total counts of pixel in the VOI do not appear in the table after I did the quantification as picture attached

You are wanting to sum the Hounsfield values of all the voxels of the scalar volume within the segmentation? How do you plan to use that statistic? SegmentStatistics already provides mean, median, min, max type values. Mean * number of voxels should give you your total sum of the corresponding Hounsfield values.