Strictly speaking I only need one pannier for my work items; however, I have a handlebar bag that I use for carrying my jacket home on the days when I need it in the morning but not the afternoon, and for change of gloves warm vs cool. In the summer, I actually need more space, for trousers & shoes to change into since I wear shorts & road shoes in the warm weather vs. regular trousers & mtb shoes in the cooler weather. So I just got a pair of small (intended as front) panniers, one of which I will use for the needed extra space.
And of course I always have my trunk bag for tire repair items, lights, and a lock.