I have 13 people assigned to tax collection with only 8 total slots for tax collectors at 2 separate manors. Every so often, the manor houses say that I'm in need of another tax collector, even though no slot has been added.
It has resulted in me having way too many people in this position because I kept promoting villagers and assigning them to the position. What's even weirder is none of them show that they're unemployed.
I started adding the people that are already assigned as a tax collector, but after a while I have to add them again. So the manor house will say that I have 3/4 tax collectors (even though I only have room for 3). I add one to fill the vacant slot, then it says that I have 3/3 and the vacancy disappears.