Hello there! That's a great question. The rarity of a 1921 silver dollar really depends on which version you have, as two different types were minted that year: the Morgan Dollar and the Peace Dollar.
For the most part, the 1921 Morgan Silver Dollar is not considered rare. In fact, it's one of the most common dates in the entire Morgan series. After not being minted for 17 years, production started up again in 1921, and over 86 million were made across the Philadelphia, Denver, and San Francisco mints. Because so many were produced, they are quite common and accessible to collectors, especially in circulated conditions. An individual 1921 Morgan only becomes rare or valuable if it is in pristine, uncirculated condition or is a specific error variety.
The coin that makes the year famous is the 1921 Peace Silver Dollar. This is the truly rare one! The Peace Dollar was first minted in late 1921 to commemorate the end of World War I. This first-year issue is special for a couple of key reasons. First, it had a very low mintage of just over one million coins. Second, it was struck in a beautiful high relief, which means the design is raised much higher from the surface than on later coins. This high-relief design proved difficult to produce and was changed to a lower relief in 1922, making the 1921 version a unique, one-year-only type.
So, to sum it up, the 1921 Morgan Dollar is extremely common, but the 1921 Peace Dollar is a celebrated key-date rarity sought after by collectors due to its low mintage and unique high-relief design.
It's worth noting that the provided search results were empty, so this answer is based on general numismatic knowledge.
