"I am working with twelve employees."
Translation:Pracuję z dwunastoma pracownikami.
Thanks for getting back to me, Vengir. My confusion arises because in the tables in the book (Polish Verbs and Essentials of Grammer / Swan) it gives both forms - -oma and -u forms as optional for instrumental for some reason, also stating "The instrumental forms in-oma are optional, alongside forms in -u" but I will make a point of not using the - u form from now on. The book wasn't written by a Polish native by the looks if things.
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.
Table: here (yes, Wiktionary, I know): https://en.wiktionary.org/wiki/dwana%C5%9Bcie#Declension
Reason: Hmmm... it seems to come from a rather outdated form of "dwa" that is still shown in this table for 2: https://en.wiktionary.org/wiki/dwa#Declension_3