The Shulchan Aruch (Orach Chayim 473:4) writes: "They bring before the head of the household a plate that has three matzot, maror, charoset, karpas or another vegetable [and vinegar or saltwater]." On this, the Magen Avraham (subsection 4) writes in the name of the Mahari"l: "It is customary to take karpas, which is an acronym for 'Samech Perach,' meaning sixty ten-thousands (600,000) worked with hard labor."