People get their lucky numbers fro a number of sources- birth dates, anniversaries, and personal lucky numbers.

On a box of Lucky Charm Cereal.

My lucky number is fourteen.

That's because I turned fourteen on the fourteenth this year. In all number games and stuff, it's served me well.

The ancient Romans had many lucky symbols, but the most common was (gasp!) a phallus. Now this might seem shocking to us, but to the ancients it was not something "dirty" but a fertility symbol. Fertility meant prosperity and general good luck. They also had numerous personal lucky charms.