I know it is the Passenger ID number. But how many digits is it. There is a 9 digit number, several spaces and then two alpha characters and 3 numerics. e.g. BC321 following the Passenger ID number.
On the back, top left of your card it says (in very small letters) PASSID/Numero d'identification. Directly under that is a 9 digit number. That's your Trusted Traveler number. It's also on the several letters you received when you first applied.
Where do I find my Trusted Traveler membership number?
Find your Trusted Traveler's membership number by logging into your Global Online Enrollment System (GOES) account. The membership number is listed toward the top of the page. The membership number, also known as your PASS ID, is also on the back of Trusted Traveler cards.
(In case you're wondering, I found that by plugging your exact question subject into Google and it was the top hit!)