I think it was only 2 people, Mildred and one of her friend
Montag headed for the river.
Montag was hiding books in 2 instances from his wife. The first instance was near the beginning after they burned the woman. He hid the books in the air vents. The second time was when Beatty came over to talk. He hid it in his pillow and Mildred wanted to fluff the pillow. References: Ray Bradbury, Fahrenheit 451, New York: Ballantine Books, 1953
There is no specific individual known as Mildred in popular culture, literature, or history. If you have more context or details, I can try to provide a more accurate response.
Toward the train tracks
Before Montag left the professor's house, he exchanged his green bullet for Faber's old, torn Bible.
Before leaving the professor's house, Montag exchanged a bible and a uncommon green-led drafting pencil.
After meeting Clarisse and how she had a different point of view on the world, he realized that he doesn't want to live in a society where it is forbidden to talk abut his thoughts and learn about anything new
In the book "Fahrenheit 451" by Ray Bradbury, Faber reads from the Book of Ecclesiastes in the Bible to Montag. Specifically, he reads Ecclesiastes 3:1-8, which talks about the different seasons of life.
Montag hid his books after the ladies left because he realized he had made a mistake by reading poetry to the ladies. He hid them in the backyard after they left.
To the river
Montag tells Faber to leave town and head towards the river where he will meet up with a group of intellectuals who are dedicated to preserving and sharing books. Faber also advises Montag to stay hidden and avoid using his real identity.