Originally Posted by An Ard Ri
Both of King Juan Carlos daughters and sisters and the youngest daughter of King Felipe VI are all Infantas of Spain.Only Leonor has the title of Princess (of Asturias).Only the heir apparent or heir presumptive bears the title Prince or Princess of Asturias,so Leonor is technically the only Princess in Spain.
The current infantas of Spain (by rank) are:
The Infanta Sofía of Spain
Infanta Elena, Duchess of Lugo
Infanta Cristina, Duchess of Palma de Mallorca
Infanta Pilar, Duchess of Badajoz,Dowager Viscountess de la Torre
Infanta Margarita, Duchess of Soria & 2nd Duchess of Hernani
Infanta Alicia,Dowager Duchess of Calabria
See this thread for further discussion
Titles of the Royal Family
It is correct that Leonor is the only one who can be addressed as "Princess Leonor".
The last lady you named however, Infanta Alicia, is twice a Princess: she is a Princess de Bourbon-Deux Siciles by marriage and a Princess de Bourbon de Parme by birth. She is the only Infanta de España who can correctly be addressed as "Princess", as also the Spanish Court does.