Why is accumulation a countable noun? I don't know how I should understand it. I canít think of any circumstance under which people say two accumulations of unwanted rubbish.
Accumulation has 2 meanings
1- The process by which something increases in amount or is collected together over time: "Slow accumulation of cholestrol in the arteries"
2- The amount of something that collects or is collected over time:"Large accumulations of snow".
As you see the second one is countable.
Resource: Macmillan dictionary for advanced learners 2003.