From the Wikipedia page 
Illán de Vacas is a town in the province of Toledo, in Castile–La Mancha, Spain. The surface area of the municipality is 9 km², it has a total population of 1 inhabitant, making it the least populated municipality in Spain.
Illán de Vacas belongs to the "comarca" of Torrijos. It is bordered on the north by the municipality of Los Cerralbos, on the northwest by Otero, on the east by Domingo Pérez, on the south by Cebolla and on the west by Lucillos.
The town lies at an elevation of roughly 480 m, and 59 km west of Toledo and 35 km east of Talavera de la Reina, on the railway line between Talavera and Torrijos. It is approximately 95 km southwest of Madrid.
With five inhabitants according to the INE census of 2017, it is the municipality with the lowest number of inhabitants of Spain together with Villarroya, in the province of La Rioja.