How can I see how many items I have in my Zotero library?

To see how many items you have, click an item in the middle pane and Select All (Command-A on macOS or Control-A on Windows/Linux). A count of selected items will appear in the right-hand pane.

To determine total items, including child attachments and notes, click an item and press the + (plus) key to expand all parent items before using Select All. You can press - (minus) afterward to collapse all items.