Not a teacher, 53-year-old American.
Whether you need an article or not really depends on the context. In your given example "I saw myriad stars." I would NOT use any article - it reads fine without it.
In a different context (from the online dictionary - link below), another example uses the article 'the'. "< the myriad problems that cities face.>"
And another example: "<the myriad activity of the new land — Meridel Le Sueur> "
Myriad - Definition and More from the Free Merriam-Webster Dictionary
Student or Learner