I say infinite because its up to you how big a number you would like displaying.
This is a single unit display that counts the number of redstone inputs it receives (how ever long or short they are).
It is designed to be pasted a number of times as it automatically links up with the previous unit if it is pasted on the stated diamond block.
[input is the gold block on the right or the schematic]
This could be used to count anything from days past (just put a daylight sense directly to the input) to the number of people to complete a particular challenge, wire a pressure plate or trip wire to the input.