How likely or rare is it that I have a brain aneurysm and will randomly die one day?

Don't worry. There are approximately 30, 000 people per year (in the US) who suffer hemorrhage from a ruptured brain aneurysm. From a general population standpoint, this can be considered a rare event. If you have a strong family history of brain aneurysms you should see a physician and perhaps be screened with an MRI of the brain.
Rare. It's extremely rare but we're all aware that rare things sometimes happen. Do you have family members with history of aneurysms or sudden, unexplained death? (in perspective, other things can cause you to "randomly die" - do you drive a motorcycle? Smoke? Scuba dive? Sky dive? Fly a private plane? Wear your seatbelt? Own a handgun? Drink and drive? Etc. Etc.).