E Pluribus Unum isn't a type of coin, it's on all government issued currency in the U.S. And there were two types of coins minted in 1921 so I really can't answer that question. Well I'll try. If the back of your coin looks like this then it is worth about $20-$30. If it looks completely different then it is worth about $120-$150.
I have a 1899 Silver Dollar What is it worth?
There are two types of 1921 silver dollar coins. One is the Morgan Silver Dollar and the other is the Peace Silver Dollar. Check both faces of the coin and that will help you to identify what coin you have. But instead of trying to sell those coins, why don't you start your own collection? It's a very interesting hobby!
Worth od E. Pluribus Unum silver dollar