"Sȳri taobi ēdrusi."

Translation: The good boys are sleeping.

[DustinArbi]

This may be a stupid question but what makes this plural? What is the indication?

[ValyrianSoGu]

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