There are a couple of wrong beliefs that from time to time arise in conversations about determining shutter count of a camera. In this little article I want to address all of them so I can stop wasting my time repeating myself and just send a link to this page instead.
Allegation 1: I can rely on image number (IMG_XXXX.CR2) to determine shutter count
Wrong! The image number is completely useless for determining shutter actuations count. There are a couple of reasons for it. First, image number is limited to four digit places. The maximum represented number is 9,999, while actual shutter resource is much bigger — it varies in 100,000–150,000+ shots range. Second, image number resets after overflow, it will start over from zero after count reaches 9,999 limit. Third, this counter can be easily reset from the camera menu. It is also unsuitable if you have more than one memory card / directory.
Allegation 2: I can rely on the shots taken counter in the «Battery» menu
Wrong! While this value advertises itself like a real shutter count it is just a number of shots that were taken from the last full battery charge. No doubt as it rarely exceeds one thousand. It is there for reason of estimation how well your battery performs and making sure you don't accidentally end up with a discharged battery in a crucial event.
Allegation 3: I can look into image EXIF metadata to read shutter count
Wrong! There is nothing in Canon images' EXIF that resembles a shutter count. While some other DSLR vendors like Nikon do really embed shutter actuations count information into the images, Canon does not (AFAIK, the only exception is the top-level 1D* cameras).
It may now sound like a statement by the Captain Obvious, but you really need a special software to retrieve shutter count from a Canon EOS camera. Luckily, there is a great app for OS X that does it for you.