...and having sales (and how many per icons online?)

Hi, thats an interesting site...however looking at the licensing page, I can't understand how you would make money because they only name a creative commons license where someone can pay you but doesnt' have to, and a public domain license?

https://thenounproject.com/handbook/upload/#curation

The answer is in there link you posted. If a buyer doesn't want to credit the author right next to the icon, they have to buy a license. There's also a subscription scheme, look here:
https://thenounproject.com/handbook/royalties/#getting_paid