In the Library view (specifically the Gallery view) it is difficult to see how many items you have selected as there is only a thin outer rectangle around each image that is drawn. And I think there are bugs with this as performing a multiple select via Shift-click and scrolling seems to lose the initial selected item.
Anyway, to remove any confusion, how about placing some text on the dialog that indicates the number of selected items? It could be located next to the total number of items text that currently exists. So something like:
Total: 15026 Entries [38150 Mb] 1 Entry Selected
Total: 15026 Entries [38150 Mb] 27 Entries Selected
Then we’ll know immediately have many are selected and can then remove them, etc.