"I have only one postage stamp."
Translation:Ich habe nur eine Briefmarke.
"Eins" does mean "one", but it is merely the name of the number, the base form if you will. You would afaik only use it when doing math.
I'm sure you know that for singular nouns you need an article, and the indefinite article in German so happens to be the word "one".
Thus German does not distinguish between "a stamp" and "one stamp" - it's the same thing.