This may be a stupid question but what makes this plural? What is the indication?
Well, pretty much everything. "taobi" is plural of "taoba", "sȳri" accordingly is the plural for "sȳz", and the singular form for "ēdrusi" would be "ēdrus".
A good place to start researching would be here: https://wiki.dothraki.org/High_Valyrian_Noun_Declensions