Minecraft wiki entry for Hopper, associated text: The top Hoppers contain only 1 diamond, which represent the item being sorted, and 21 cheap items stackable in the other slots. (Renaming the 21 items is recommended.)
18
1-wide, tileable, silent, transfer rate: 2.5 ips.
overflow protected only with part "h-A" and "h-B" mixed in alternating order.