singulus

Singulus, singula, singulum is first and second declension, but in practice it lives in the plural: singuli, singulae, singula. It belongs to the distributive numerals - the series that answers "how many each" rather than "how many" - alongside bini (two each), terni (three each) and quaterni. So singulis militibus denarius datus est means every soldier got one denarius, not that a single soldier got the lot.

That is the trap in one line: singuli is not unus and not solus. Its other job is to itemise, "one by one, taken separately", which is where per singulos dies, "day by day", comes from. This site also lists the plural singuli as its own entry because that is the form the corpus actually uses. English single, singular, singleton and singly all descend from it.