There are a gazillion gazillion of them. I know some who like to collect them as full sheets. If you look at the Scott catalog, most them still catalog at Scott minimum value for mint stamps. But they are definitely fun to have!
The used stamps (especially on cover) are often more expensive to buy. You will note that Scott often italicizes those used prices. Those prices are for genuine postally used stamps. Many so-called used stamps from that era were not genuinely postally-used, but either favor-cancelled, CTOs, or fake-cancelled with genuine postal cancelling devices -- these stamps are not worth any more than the mint stamps, so proceed with caution if you plan on acquiring used stamps (don't be fooled thinking that you are getting a huge discount off the used price). So if you have a genuinely used example on cover, it will have a noticeable premium.
Just remember, collect for the fun of it (consider any additional financial value as simply a bonus) -- my opinion.

k