Get info on the file in the Finder. Open up the More Info section, then select and copy all the layer names to the clipboard.
Open up a new TextEdit document and paste the layer names into the document.
Press Command-F to open the Find pane and type “, ” (a comma and a space). The light grey text to the right of the input field will let you know the total number of layers and groups, minus one, in your Photoshop document (minus one, because the last name doesn’t have a comma after it).
This document has 5 layers. Please note that groups are counted as well.
There is one caveat — layers that have “, ” in their name will be counted twice. Who uses commas in their layer names though?
PSDs on networked drives don’t show a list of layer names in the Finder’s Info window, so you’ll need to copy the file to a local drive before being able to see how many layers have been used.