Twenty examples is a lot to ask for, but I will give you six, and hopefully you can find the rest yourself.

Montag uses a lot of similes to describe Clarisse. For example, he says that her face is as "bright as snow in the moonlight," implying that she is radiant and that she has a beauty which seems to glow. Montag also describes Clarisse's face as "like the dial of a small clock seen faintly in a dark room in the middle of a night." This simile seems to suggest, again, that Clarisse represents a glimmer of light in the darkness and perhaps a promise of better, brighter times to come. One of the reasons Montag is attracted to Clarisse is because he sees in her much of himself and because she makes him reflect on his own life. This is perhaps why Montag says, again using a simile, that her face is also "like a mirror."

Bradbury also uses similes to describe the protagonist, Montag. For example, Bradbury writes that Montag "wore his happiness like a mask." The implication here is that the happiness that Montag projects is a fabrication and a pretense. He wears this mask because it is safer to appear happy. Beneath the mask, however, he is of course not very happy at all.

About a third of the way through the story, there is a passage describing Mildred, Montag's wife, in a particularly sinister manner. Her body is described, using a simile, to be "as thin as a praying mantis from dieting," and her "flesh" is described as "like white bacon." Mildred personifies society's standards of beauty. Mildred starves herself to look thin, and she seems pallid and ill as a consequence.

Wow-that's quite a tall order.  Have you read through the book and tried to find some yourself?  To clarify, a simile is when you compare two things using the words like or as.  Look to the very beginning of the book, when Montag meets Clarisse, and you will be able to find quite a few; Bradbury waxes quite poetical when describing Clarisse.  I'll jot down a couple, to get you started.  In describing Clarrisse's face after Montag gets home after meeting her for the first time, Bradbury writes,

"She had a very thin face like the dial of a small clock seen faintly in a dark room in the middle of the night...with a white silence and a glowing, all certainty and knowing."

This relays the fact that Montag feels assured and comforted by Clarisse; she is knowing, and a beacon of assurance in a dark world.  Bradbury goes on with another simile for Clarisse's face:  "how like a mirror, too, her face," meaning that she had the ability to help people see themselves for who they really are.  Again, on Clarisse:

"she was like the eager watcher of a marionatte show, anticipating...before it began,"

meaning that she is an observer of their world, curiously watching what is going on, and able to understand everyone's movements.  Bradbury uses similes all throughout his novel, and if you keep your eyes open for them, they almost always convey a deeper symbolism for the characters and the moments they experiences.  Keep flipping through the pages; he uses them to describe Mildred also, and there are quite a few at the end of the book.  I hope that helps a bit!