They are sold in the various sizes listed. They are measured by length and each one has unique markings with no two exactly the same.
The number of tubes on each one will vary and may be epoxy molded or glued together to help hold them together on the larger ones depending on stock at the time of order. Please note on the larger 7"+ Barnacles they may vibrate free or come apart in shipping and can be easily glued back together again with a hot glue or crazy glue.
These are a beautiful species to add to any collection.