You might find it meaningless about knowing how you can find your Cydia Account Number but you should realize that it's really important for you to know it especially for give-aways.
The below video will show how easily you can find your Cydia Account Number by just going through three simple steps.
Finding Your Cydia Account Number in just 3 Simple and Easy Steps
Step 1: Open Cydia (Your device should be jailbroken).
Step 2 : Click the Manage Account on the main Cydia Tab and by using Google or Facebook, verify your account. You are likely to have one if you have made purchase of any app or tweak via Cydia.
Step 3 : Next below the 'Installable Purchase' panel you will see your Cydia Account Number in small text once you've logged in. For reference in future, jot it down.
When you are in to participate in a jailbreak app or tweak giveaways, it's helpful to know it. Thus its important to know it!