When Pond5 used to show sales figures on item pages, I remember checking out the number one selling video clip and it was about 1000 sales over a period of three years. So that would make it roughly one download a day. Sales were probably higher earlier on though, so two a day would probably be more likely. I'm not sure what the sales are like on Shutterstock/iStock/Fotolia compared to Pond 5, but I'd be surprised if they are ten times more.
So although 10 to 20 a day is probably feasible, I think you could probably count on one hand the amount of clips that have achieved that, and I'm not sure how sustainable that would be.
0.5 sales a day over a ten month period is my best result.