Option 1: Use a bookmarklet. Drag the Word Count bookmarklet in the centre of this page to your bookmarks bar. Select some text and click the bookmarklet. If you don’t use ⌘-1 through ⌘-9 to switch tabs (deselected in Safari > Preferences > Tabs, you can use ⌘ and the number corresponding to the ordinal position of the bookmarklet from left to right in your bookmarks bar. So if the bookmarklet is second from the left the keyboard shortcut would be ⌘-2. Hit the ESC key to dismiss the modal popup.
Option 2: Install the free WordService by DEVONtechnologies from the Mac App Store. Included is the Statistics service which you access from the contextual menu when you Control-click or right-click and scroll to the Services menu item. You can also set a keyboard shortcut by navigating to > System Preferences > Keyboard > Shortcuts > Services. The advantage of this option is that it is system-wide.