question

The theoretical limit for the number of elements in a set collection is two billion. In practical terms, you should only store tens or hundreds of elements in a collection. To be clear, it is possible to store lots more in a set but storing thousands of elements isn't ideal.

NOTE: The limit used to be capped at MAX_UNSIGNED_SHORT (64K):

        if (bytes.remaining() > FBUtilities.MAX_UNSIGNED_SHORT)
            throw new InvalidRequestException(String.format("Set value is too long. Set values are limited to %d bytes but %d bytes value provided",
              FBUtilities.MAX_UNSIGNED_SHORT,
              bytes.remaining()));

but the cap has been removed since C* 3.0.1, 3.1 (CASSANDRA-10374).

CQL collections are designed to store small amounts of data such as a person's phone numbers or addresses, or tags/labels in a product catalogue. They are not designed for unbounded datasets.

When reading the contents of a CQL collection, the entire collection is retrieved even when you just want to read one element because the collection is not indexed. For this reason, you should model your data differently and instead store them as clustering columns if you intend to have large amounts of data in a set. Cheers!