how many 7 segment LEDs can a arduino run?

If you look at this page:-
http://www.andreadrian.de/schach/#Selbstbau_Schachcomputer_SHAH
You will see the schematic has four 7 segment displays multiplexed. The Text is about a chess computer using an arduino and is in German but fortunately the diagram is in schematic an I read that very well. :wink: