King Juan Carlos and his family have a number of Royal Palaces, sites and homes at their disposition.
In practice, however, they only use a fraction of them on a regular basis, and mostly for official occasions. Below is a non-exhaustive list:

The Madrid Royal Palace, above, is the King’s official residence and site for the most important state gatherings.
It has a large dining room, numerous works of art, an armory with historic weapons belonging to members of the Royal Family, and the official Throne Room.

The Royal Site of San Lorenzo del Escorial is a large palace and monastery in the Madrid region, which includes a tapestry museum, a library, a basilica and the official burial place for members of the Spanish Royal Family.
On the grounds is a small house called La Casita del Principe for the Prince of Asturias, the heir to the Spanish throne.
Nearby is the Valley of the Fallen, resting place for former Dictator Francisco Franco.
Prince Felipe and his wife Princess Letizia, however, currently live in a newly built small palace often called the Prince’s Pavilion, pictured below.
The Prince’s house is a short drive from Zarzuela Palace, where King Juan Carlos and Queen Sofia have lived for several decades.
Former Franco rehabilitated Zarzuela, an old royal hunting lodge, for the young couple. They moved in in shortly after their wedding in Athens, when their future as heirs to the Spanish throne was still uncertain.

The King and other members of the Royal Family often receive informal visits at Zarzuela, which is equipped with communications capabilities and contains Royal Household offices in support of the Monarch as the Head of State.
The King’s dispatch is at Zarzuela with his official Royal Standard flying above. Queen Sofia’s sister, Princess Irene of Greece, also lives on the site.
The Zarzuela Palace, also outside of central Madrid, is in the grounds of El Pardo Palace, pictured above. Franco used to live at El Pardo, which now includes reception rooms and apartments for guests.
Most Royal Palaces and sites, which include monasteries and gardens, are owned by the state and run by Patrimonio Nacional or National Heritage.
Also in the Madrid region, but to the south, is the Royal Palace of Aranjuez, pictured below.
Several Royal sites are located in the Segovia area, north of Madrid.
The Gardens of La Granja de San Idelfonso, above, extend more then 1,000 acres. The Riofrio Palace, below, was built for Queen Elisabeth Farnese in the 1700’s but has generally been barely used.
Also in Segovia is the Alcazar de Segovia, which was used by Queen Isabella of Castile. It is run by a patronage and one of several castles that were once royal homes are now museums.
The King’s official residence in Barcelona is owned by the local government. The same goes for the Royal Family’s summer residence in the Balearic Islands, Marivent Palace in Palma de Mallorca.

© Agencia EFE
However, they also use another site down the coast in Palma run by Patrimonio Nacional. Almudaina Palace, below, was once an Arab fort.
Then there’s the Royal Residence at La Mareta in the Canary Islands. It was a gift of King Hussein of Jordan to King Juan Carlos.