I use a Chrome Extension called Official card counter trello with Totaliser. It gives you a number per Column and a total for the board.
This extension is not safe. It sounds malicious in that it redirects you to affiliate URLs while shopping online and uses a variety of other trackers. Beware. Read the reviews.
A much easier way is to simply press the keyboard shortcut "Q" while looking at your board. This minimizes your lists and shows the total number of cards.
Wow! Didn't realize. I'll remove it from mine and alert our other users. I knew about hitting the letter "q" but rarely use it as I need to see all. Guess I could get used to toggling if I really want to see the total number of cards.