A finished item is any item that has reached the final SRS stage. Items that are finished will no longer show up in your reviews. This is because an item that becomes finished is already memorized, and no longer needs to be reviewed!
You can find the number of items you have finished at any time within the SRS stage distribution section of the dashboard.
Any item that has reached the final stage will be considered a Diamond item, so the number of Diamond items you have is simply the number of finished items you have completed!