If you do it bay hand at the command line , it will show at the command line.
If you do it by LISP , guess
(setq selection ( ssget "c" pt1 pt2 ))
(setq selection quantity (sslenght selection))
But pt1 and pt2 SHALL be at the visible screen, so before you must do a ZOOM window pt1 pt2 . previous to SSGET , as follow
(command "zoom" "w" pt1 pt2 )
(setq selection ( ssget "c" pt1 pt2 ))
(setq selection quantity (sslenght selection))
The only SSGET that select all is (ssget "X" )
X Entire database. If you specify the X selection method and do not provide a filter-list, ssget selects all entities in the database,
including entities on layers that are off, frozen, and out of the visible
screen.