At the top of the drive/folder list on the left side of the library module, Catalog tab, is 'All Photographs'. That number shows all of the images in the catalogue, independent of where they are. Click on that before filtering. If you're searching based on a specific keyword, you can also open the keyword list in the right panel of the library and there will be a number beside each keyword showing the number of images that have that keyword. Place your mouse over a keyword and to the right of that number there will be an arrow. Click on the arrow and it will show you a filter with only images containing that keyword. Creating a Smart Collection would also work (don't forget the Alt/Opt key when creating Smart Collections).