If your animations have 8 frames and you want them all across one row, then the number of columns should match your number of frames. So you enter 8. Let the width auto-calculate. 384 comes from your width of 48px * 8 frames = 384px
In your case you can just choose the “horizontal strip” option. The “By Rows” and “By Columns” options are if you want to specify the number of rows or columns, creating a “matrix” of frames.