I use one pannier. Clothes I need, any food, wallet etc all goes in there. Two bottle cages, one for water bottle and one for coffee thermos. Plenty of sealed containers available to haul food/drinks without spilling.
Office has a fridge/freezer and I'm on the road a lot at work, so stops to the grocery store happen then to keep that "stocked", but mostly I just drink water.
Spring & fall you need extra riding clothes or less clothes when the temps might swing from 30F - 70F over the day, so it fits the extras too. Some days it's almost empty. some days it's packed full.

The rack also lets me use a smallish trunk bag for non commute rides when I want to bring stuff. It's also handy for those roadside shopping unexpected finds. I've been known to strap cast iron grates for my grill, a roll of landscape fabric, scrap steel stock, whatever I might find that fits. Then I don't have to go back with the car. You'd be surprised some of the things I find in the road that will come home with me.