Most numerology numbers come from a birth date, which is fixed the moment you're born. The destiny number takes a different ingredient entirely: your name, letter by letter, exactly as it appears on your birth certificate.
Each letter is assigned a numeral — A is 1, B is 2, and so on, though the exact mapping depends on which school you're using. Every letter's value gets added up, then reduced through digit-sum arithmetic until one core number remains.
The name "destiny number" is doing some rhetorical work here — it implies fixed fate rather than, say, "expression number," which is the more common academic-sounding label for the same calculation. Both names refer to the identical arithmetic; only the marketing differs.