David has joined the Whites but his squad number has not been revealed yet.
However, he won't be able to wear it for Real Madrid: La Liga registration rules state that first-team players have to take up numbers from 1 to 25.
In the Austrian national team, Alaba plays with '8' on his back. However, number 8 is occupied at Madrid, Toni Kroos wears it.
These numbers are currently available: 16, 18, 21.