"There is an increase of the quantity."
Translation:Υπάρχει αύξηση της ποσότητας.
This sentence sounds quite like a scientific remark, so can definitely use "μία αύξηση" instead of just "αύξηση". In any case it is definitely not wrong to use the article, just depends on the context -which of course is missing here so makes this a bit more tricky :-)
Your point is well taken and it does sound better with "μία" unfortunately, "μία" ισ not in the original sentence which is locked so we can't edit it.