winning number; lucky number
Refers to the number that wins a lottery, raffle, or similar draw. 'Lucky number' is a looser translation; the core meaning is the drawn winning number.
The winning numbers for the lottery were announced.
Please check the winning number.
当選 means 'being elected' or 'winning a prize'; 当選番号 specifically refers to the winning number itself.
番号 is a general 'number'; 当選番号 is a compound meaning 'winning number'.
Compound of 当選 (winning, being elected) and 番号 (number).