Your European Youth Card number has 16 characters (letters and numbers). You can find it on your card in the DiscoverEU Travel App: go to More > DiscoverEU European Youth Card.
You'll mostly need your card number for discounts that need to check your card, especially online discounts during your DiscoverEU trip.
Good to know: your card is available in the DiscoverEU Travel App while your DiscoverEU Interrail Pass is valid. Once your Pass expires, you'll no longer see your card in the app.