There were some features/layout options from vBulletin that are not on Xenforo by default. The top 20 posters (along with the other overview statistics) are the forum defaults. Theoretically, other and/or more data could be shown with an additional add-on but would likely be an additional charge - especially if it is something that does not exist currently.
I asked Jer (and then accidentally fell asleep) if these could be increased. It is a possibility with the ‘Richest Users’ panel under Transactions.
We have talked and will soon welcome other features but this isn't something on the immediate radar.
Seashells were introduced back in...2016 I believe? Seashells are a currency alongside bells to be used in ‘Kapp’n’s Seaside Shack Shop.’ They were given to new users to make purchasing some forum add-one like name changes or user title color items more accessible. This was later extended to those that had been members on The Bell Tree for a minimum of two years with at least 20 posts.
They are not given out currently as they are not set up but will eventually return.