I knew a little about this rule, had the idea that Marcelo and Higuain would be considered as homegrown players, but there was more to it so I did a little research and found out that Real Madrid are more than covered in the homegrown department.
This is how UEFA defined homegrown players on their website:
UEFA defines locally-trained or 'homegrown' players as those who, regardless of their nationality, have been trained by their club or by another club in the same national association for at least three years between the age of 15 and 21. Up to half of the locally-trained players must be from the club itself, with the others being either from the club itself or from other clubs in the same association. The UEFA rule contains no nationality conditions whatsoever.According to this definition, these players can be called homegrown players: Iker Casillas (GK), Sergio Ramos (DF), Esteban Granero (MF), Marcelo (MF) , Antonio Adan (GK), Xabi Alonso (MF), Sergio Canales (MF), Alvaro Arbeloa (DF), Raul Albiol (DF), Ezequiel Garay (DF)*, Gonzalo Higuain (FW), and Pedro Leon (MF).
*I am not too sure about Garay, but he did complete 3 seasons with Racing Santander before he turned 21, so he can be considered homegrown.
The ones in bold font have completed 3 seasons between the age of 15-21 in the Liga BBVA with different teams, but will be classified as homegrown players because of the condition stated above. The rest of them have completed 3 seasons with Real Madrid between the age of 15-21.
UEFA have set a limit of 25 players to be used in the European competitions, and out of these 25, eight of the players have to be homegrown. Real Madrid currently have 12, out of which 7 are trained by Real Madrid, thus also fulfilling the criteria that 50% of the homegrown players should be trained by the club they represent.
A few of the players are going to be sold, and if Real Madrid fall short in the homegrown department which is highly unlikely, they can name a few players of the academy in the list of 25, as there is no rule that obliges the club to play a certain number of homegrown players on the field.
So there you have it, Real Madrid are well covered in the homegrown department, I hope this clears the confusion created by the rule. Hala Madrid!
If you want to read more about the rule, visit UEFA's Official Web-Site.