quinque

Quinque is indeclinable, like every cardinal from four to a hundred, so it keeps one shape whatever case its noun happens to be in. It governs nothing: the noun takes whichever case the sentence needs, and quinque just sits there.

The rest of the family does decline. The ordinal is quintus, the distributive quini, "five each", and the adverb quinquiens, "five times". The Roman numeral V is the sign, and the derivatives run a long way: quinquennium, five years; quinquaginta, fifty; Quintilis, the old name for July before it was renamed after Julius Caesar; and, through French, quintet and quintessence.