There is no built-in option to reference just the number of a caption.

What you can do is redefine the hidden bookmark inserted by Word. To
do this, first insert the cross-reference, to "Only label and number"
(this creates the hidden bookmark). Then carefully select just the
caption number. On the Insert menu, click Bookmark. In the Bookmark
dialog box, check the "Hidden bookmarks" option, and then click the
"Location" radio button. The bookmark which includes the caption
number will be displayed at "Bookmark name" at the top of the dialog
box. Click the Add button. Finally, update the cross-reference by
selecting it and pressing F9.

Alternatively, manually insert a bookmark (via Insert | Bookmark)
around the caption number and then insert a cross-reference to that
bookmark.