Hi. Donut is an adjective of the shop and adjectives are never plural. Another example: a 100 yen donut, where 100 yen is an adjective, so not yens. I hope that explains it for you. I'm impressed that you ate 10 - I love donuts and I'm a big guy, but I think I'd be struggling to eat that many. I hope they were good. Sometimes we need to just enjoy life!

Well done