We put a comma after introductory words ; now introduces your sentence
"An adverb or adverb phrase that begins a sentence must be followed by a comma if it serves as a link to the previous sentence."
"She believed he had the talent to succeed. Unfortunately, he was lazy."
source:English 2126: Modern English Grammar: The Punctuation Handbook
You have two previous sentences-Close your eyes and put your finger on any letter. Open your eyes.] before the sentence starting with now:Now, each one of you has one letter.
Furthermore, some grammarians say that the comma is optional for the following adverbs: then, so, yet. Example:
Then, she ran upstairs. Then she ran upstairs.
Punctuation - Comma (,) - Grade 5 - English Language - kwizNET Math/Science/English Homeschool/Afterschool/Tutoring Lessons/Worksheets/Quizzes/Trivia
My opinion : It is usual to use a comma when an adverb is used as an introduction to a sentence.