"I am working with twelve employees."
Translation:Pracuję z dwunastoma pracownikami.
I, the course creators, and some more people had a discussion about this. The conclusion we've come up to is that the -u form in instrumental is best avoided, regardless of whether grammar books allow it or not. Because even though we did find some phrases with the -u form in instrumental, it's not used commonly overall and is thus considered incorrect by some.