Translation:I work on Saturdays.
I think it can mean either one. If the sentence is as it is now, then you'd have to rely on context and/or how well you know the person (if you know the person pretty well, then you probably know their work schedule, and if they normally don't work on Saturdays, and they say this sentence, then it probably means just this Saturday, as you said. If you don't know their work schedule, then you'd just have to rely on context alone). I do think that if the speaker is speaking to someone who obviously doesn't know their schedule, then they'll hopefully alter the sentence to be a little more specific, like adding "this week" in there.
Anyway, I do think that the Japanese sentence is a little too vague.
Well, I am no japanese speaker, but from what I understand from some comments throughout dualingo, the "ni" particle marks a specific time or direction, while the "wa\ha" particle refers to the subject.
It's possible to deduce that it means I will work on saturday, but not how many saturdays, it seems japanese is contextual so it can be both meanings.