It is not the custom of the language to say the word אחת (one) without the thing which it is counting; as it will always say פעם אחת (one time) (unless it is in a place where the thing which it is counting is known through the noun or the verb that accompanies it).

And when it is written וכפר אהרן על קרנותיו אחת בשנה (And Aaron shall make atonement upon the horns of it once in the year)¹⁴⁶, the language is peculiar. And they interpret the word אחת to go with the verbוכפר , and that it means to say וכפר אחת (And make atonement once).

footnotes: ¹⁴⁶ Exodus 30:10.