When it's early and there's a big selection, I'll definitely take one that I like.
A couple hours later, it's pretty likely there will be unclaimed donuts. If there's another one I like, I'll take a second donut. If there are still donuts at lunchtime, I'll probably take a third if there's one that I like.
I can easily resist donuts I'm not crazy about, but no self-control if any of my favorites left. Which for me pretty much applies to all food.
Kind of related: the company that stocks the vending machines switched brands of the cakes. I was good for at least two purchases a week of something - donuts or twinkies or hohos, stuff like that. I didn't buy candy bars or pretzels or chips. Now the new brand of pastries ("Mrs. Freshley's) is terrible, and I eat none. That's a plus.