mil

There is no unu mil in Esperanto and no article either: mil homoj already means "a thousand people". Multiples simply stack in front, du mil, kvin mil, and the numeral itself never takes -j or -n, so it is mil librojn with the ending on the noun alone. Turn it into a noun with -o and it starts behaving like one: milo da soldatoj, and miloj da homoj for the vague English "thousands". -a gives the ordinal mila, -obl- gives milobla for thousandfold.